Merged "This webpage has a redirect loop"

It was easy!
I just removed Easy Links (and/or idk) mod_rewrite

Hello everyone !
Can anyone help with my website please ? I have a fresh 3.8 joomla website with a 5.1.2 kunena install.
My website : www.4u2learn.fr/joomla/4u2learn
I did not create any categories for now. I'm using the sample categories. I double checked everything (trying to see if there were no conflicting aliases in particular), uninstalled the Kunena menu, recreated it, uninstalled kunena, reinstalled. Same issue when clicking on most categories :
The page isn't redirecting properly
Firefox has detected that the server is redirecting the request for this address in a way that will never complete.
This problem can sometimes be caused by disabling or refusing to accept cookies.

What is the cause of the issue for the sake of me ? I've been tweaking so many things for the last past hours. And still nothing. I used to have a 2.5 joomla website working with kunena 3.0.2, tried to replicate the same config in my new website, in vain. Most categories links crash (why not the first category but all the others ???). I also read the 50 pages of this forum plus tens of others. In vain again. I also checked the links for the global configuration of Kunena as well as the SEO settings. Basically they are the same as in my former webiste. I'm desperate for a solution. If anyone could just have a look and give me a clue.
Thanking you in advance for the time spent looking at this frustrating issue.
Fabio

For the Kunena config :

Database collation check: The collation of your table fields are correct

Joomla! SEF: Enabled | Joomla! SEF rewrite: Enabled | FTP layer: Disabled |
htaccess: Missing | PHP environment: Max execution time: 300 seconds | Max execution memory: 512M | Max file upload: 128M

Kunena menu details:

Warning: Spoiler!

ID	Name	Menutype	Link	Path	In trash
304	Forum	kunenamenu	view=home&defaultmenu=306	forum	No
305	Index	kunenamenu	view=category&layout=list&catid=1	forum/main-forum	No
306	Recent Topics	kunenamenu	view=topics&mode=replies	forum/recent	No
307	Unread	kunenamenu	view=topics&layout=unread	forum/unread	No
308	New Topic	kunenamenu	view=topic&layout=create	forum/newtopic	No
309	No Replies	kunenamenu	view=topics&mode=noreplies	forum/noreplies	No
310	My Topics	kunenamenu	view=topics&layout=user&mode=default	forum/mylatest	No
311	Profile	kunenamenu	view=user	forum/profile	No
312	Help	kunenamenu	view=misc	forum/help	No
313	Search	kunenamenu	view=search	forum/search	No

Joomla default template details : g5_helium | author: RocketTheme, LLC | version: 5.4.24 | creationdate: July 4, 2018

Kunena default template details : Crypsis | author: Kunena Team | version: 5.1.2 | creationdate: 2018-07-15

Kunena template params:

Warning: Spoiler!

DefaultCategoryicon
profileIconset	default
DefaultIconset	default
editorIconset	default
bootstrap	0
icomoon	1
tooltips	1
borderless	1
SubjectLengthMessage	80
fullactions	1
quick	0
displayMenu	1
displayDropdownMenu	1
displayDropdownContent	1
displayModule	1
displayBreadcrumb	1
displayAnnouncement	1
displayFooter	1
avatarPosition	left
avatarType	img-circle
labels	0
whoisonlineName	default
formRecover	0
topicicontype	B2
fontawesome	0
socialshare	0
socialsharetag
socialtheme	classic
optional_username	0
writeaccess	0
localstorage	0
IconColor	inherit
IconColorNew	#48a348
editor	0
video	1
maps	1
emoticons	1
twitter	1
ebay	1
link	1
picture	1
hide	1
spoiler	1
table	1
code	1
quote	1
divider	1
instagram	1
soundcloud	1
confidential	1
hr	1
listitem	1
supscript	1
subscript	1
numericlist	1
bulletedlist	1
alignright	1
alignleft	1
center	1
underline	1
italic	1
bold	1
strikethrough	1
colors	1
size	1
wysibb
avatarSizeX	40
avatarSizeY	90
avatarSizeXThumb	36
avatarSizeYThumb	36
avatarSizeXWelcome	72
avatarSizeYWelcome	72
avatarSizeXList	36
avatarSizeYList	36
avatarSizeXPost	144
avatarSizeYPost	144
avatarSizeXProfile	200
avatarSizeYProfile	200
templatebyText
templatebyName
templatebyLink

Kunena version detailed: Kunena 5.1.2 | 2018-07-15 [ Charon ]
| Kunena detailed configuration:

Warning: Spoiler!

Kunena config settings:
board_offline	0
enablerss	1
threads_per_page	20
messages_per_page	6
messages_per_page_search	15
showhistory	1
historylimit	6
shownew	1
disemoticons	0
template	crypsis
showannouncement	1
avataroncat	0
catimagepath	category_images
showchildcaticon	1
rtewidth	450
rteheight	300
enableforumjump	1
reportmsg	1
username	1
askemail	0
showemail	0
showuserstats	1
showkarma	1
useredit	1
useredittime	0
useredittimegrace	600
editmarkup	1
allowsubscriptions	1
subscriptionschecked	1
allowfavorites	1
maxsubject	50
maxsig	300
regonly	0
pubwrite	0
floodprotection	0
mailmod	0
mailadmin	0
captcha	0
mailfull	1
allowavatarupload	1
allowavatargallery	1
avatarquality	75
avatarsize	2048
imageheight	800
imagewidth	800
imagesize	150
filetypes	txt,rtf,pdf,zip,tar.gz,tgz,tar.bz2
filesize	120
showranking	1
rankimages	1
userlist_rows	30
userlist_online	1
userlist_avatar	1
userlist_posts	1
userlist_karma	1
userlist_email	0
userlist_joindate	1
userlist_lastvisitdate	1
userlist_userhits	1
latestcategory
showstats	1
showwhoisonline	1
showgenstats	1
showpopuserstats	1
popusercount	5
showpopsubjectstats	1
popsubjectcount	5
showspoilertag	1
showvideotag	1
showebaytag	1
trimlongurls	1
trimlongurlsfront	40
trimlongurlsback	20
autoembedyoutube	1
autoembedebay	1
ebaylanguagecode	en-us
sessiontimeout	1800
highlightcode	0
rss_type	topic
rss_timelimit	month
rss_limit	100
rss_included_categories
rss_excluded_categories
rss_specification	rss2.0
rss_allow_html	1
rss_author_format	name
rss_author_in_title	1
rss_word_count	0
rss_old_titles	1
rss_cache	900
defaultpage	recent
default_sort	asc
sef	1
showimgforguest	1
showfileforguest	1
pollnboptions	4
pollallowvoteone	1
pollenabled	1
poppollscount	5
showpoppollstats	1
polltimebtvotes	00:15:00
pollnbvotesbyuser	100
pollresultsuserslist	1
maxpersotext	50
ordering_system	mesid
post_dateformat	ago
post_dateformat_hover	datetime
hide_ip	1
imagetypes	jpg,jpeg,gif,png
checkmimetypes	1
imagemimetypes	image/jpeg,image/jpg,image/gif,image/png
imagequality	50
thumbheight	32
thumbwidth	32
hideuserprofileinfo	put_empty
boxghostmessage	0
userdeletetmessage	0
latestcategory_in	1
topicicons	1
debug	0
catsautosubscribed	0
showbannedreason	0
showthankyou	1
showpopthankyoustats	1
popthankscount	5
mod_see_deleted	0
bbcode_img_secure	text
listcat_show_moderators	1
lightbox	1
show_list_time	720
show_session_type	2
show_session_starttime	1800
userlist_allowed	1
userlist_count_users	1
enable_threaded_layouts	0
category_subscriptions	post
topic_subscriptions	every
pubprofile	1
thankyou_max	10
email_recipient_count	0
email_recipient_privacy	bcc
captcha_post_limit	0
image_upload	registered
file_upload	registered
topic_layout	flat
time_to_create_page	1
show_imgfiles_manage_profile	1
hold_newusers_posts	0
hold_guest_posts	0
attachment_limit	8
pickup_category	0
article_display	intro
send_emails	1
fallback_english	1
cache	1
cache_time	60
iptracking	1
rss_feedburner_url
autolink	1
access_component	1
statslink_allowed	1
superadmin_userlist	0
legacy_urls	1
attachment_protection	0
categoryicons	1
avatarresizemethod	1
avatarcrop	0
user_report	1
searchtime	365
teaser	0
ebay_language	0
allow_change_subject	1
max_links	6
read_only	0
ratingenabled	0
url_subject_topic	0
log_moderation	0
attach_start	0
attach_end	14
attachment_utf8	1
autoembedsoundcloud	1
emailheader	media/kunena/email/hero-wide.png
user_status	1
signature	1
personal	1
social	1
plain_email	0
moderator_permdelete	0
avatartypes	gif, jpeg, jpg, png
smartlinking	0
defaultavatar	nophoto.png
defaultavatarsmall	s_nophoto.png
quickreply	1
avataredit	0
activemenuitem
mainmenu_id
home_id
index_id
moderators_id
topiclist_id
misc_id
profile_id
search_id
avatar_type	1
sef_redirect	1
allow_edit_poll	1
use_system_emails	0

| Kunena integration settings:

Warning: Spoiler!

Kunena - Finder Disabled
Kunena - AltaUserPoints Disabled
Kunena - Community Builder Disabled
Kunena - Easyblog Disabled
Kunena - Easyprofile Disabled
Kunena - Easysocial Disabled
Kunena - Gravatar Disabled
Kunena - JomSocial Disabled
Kunena - Joomla Enabled: access=1 login=1
Kunena - Kunena Enabled: avatar=1 profile=1
Kunena - UddeIM Disabled

| Joomla! detailed language files installed:

Warning: Spoiler!

Joomla! languages installed:
fr-FR	French (FR)
en-GB	English (en-GB)
en-US	English (en-US)

Third-party components: None

Third-party SEF components: None

Plugins: None

Modules: None

Well, next episode (but not last ?)
I decided to revert to an older version of kunena (3.0.0), one which worked perfectly on my former website. Well, fantastic, everything works anew ! so, I moved on to 5.0.14. Works plain-sailing as well, but there is an icon problem for some categories : in my templates in version 5.1.2, some icons were displayed in green (most probably because topics had been consulted once ?), well, in version 5.0.14, the icons do not display at all !
Anyway, I think I just uncivered a bug in version 5.1.2. If any developer would like to contact me , I would be delighted to test, replicate and help ! One thing i know for sure is that the categories which produce this error page have no topics inside. And there's also the "main forum" link which does not work.
To be continued
Fabio