Important ERROR 500 when users log in, was working fine before upgrade to 2.0.1

I'm getting desperate now. Everything was working ok but I've upgraded to 2.0.1 and now when a registered user tries to log in I get this error: error 500

You may not be able to visit this page because of:

an out-of-date bookmark/favourite
a search engine that has an out-of-date listing for this site
a mistyped address
you have no access to this page
The requested resource was not found.
An error has occurred while processing your request.

If a super user tries to log in a bar comes up on the page saying
JUser: :_load: Unable to load user with ID: 97





Config report:

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

Legacy mode: Disabled | Joomla! SEF: Enabled | Joomla! SEF rewrite: Disabled | FTP layer: Disabled |

This message contains confidential information

htaccess: Missing | PHP environment: Max execution time: 30 seconds | Max execution memory: 128M | Max file upload: 2M

Kunena menu details:

Warning: Spoiler!

ID	Name	Menutype	Link	Path
121	Forum	kunenamenu	view=home&defaultmenu=122	forum
122	Index	kunenamenu	view=category&layout=list&catid=0	forum/index
123	Recent Topics	kunenamenu	view=topics&mode=replies	forum/recent
124	New Topic	kunenamenu	view=topic&layout=create	forum/newtopic
125	No Replies	kunenamenu	view=topics&mode=noreplies	forum/noreplies
126	My Topics	kunenamenu	view=topics&layout=user&mode=default	forum/mylatest
127	Profile	kunenamenu	view=user	forum/profile
128	Help	kunenamenu	view=misc	forum/help
129	Search	kunenamenu	view=search	forum/search
130	Parent's Forum	mainmenu	Itemid=121	kunena-2012-06-28

Joomla default template details : ifreedom-fjt | author: FJT | version: 2.5.0 | creationdate: 12-04-2012

Kunena default template details : Blue Eagle 2.0 | author: Kunena Team | version: 2.0.1 | creationdate: 2012-07-07

Kunena version detailed: Kunena 2.0.1 | 2012-07-07 [ Balozi ]
| 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	blue_eagle
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	0
subscriptionschecked	1
allowfavorites	1
maxsubject	50
maxsig	300
regonly	0
changename	1
pubwrite	0
floodprotection	0
mailmod	1
mailadmin	1
captcha	0
mailfull	1
allowavatarupload	1
allowavatargallery	0
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_name	1
userlist_posts	1
userlist_karma	1
userlist_email	0
userlist_usertype	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
usernamechange	1
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
sefutf8	0
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
version_check	1
showthankyou	1
showpopthankyoustats	1
popthankscount	5
mod_see_deleted	1
bbcode_img_secure	text
listcat_show_moderators	1
lightbox	1
show_list_time	720
show_session_type	0
show_session_starttime	0
userlist_allowed	0
userlist_count_users	1
enable_threaded_layouts	0
category_subscriptions	post
topic_subscriptions	every
pubprofile	0
thankyou_max	10
email_recipient_count	95
email_recipient_privacy	bcc
captcha_post_limit	0
keywords	0
userkeywords	0
image_upload	registered
file_upload	registered
topic_layout	flat
time_to_create_page	1
show_imgfiles_manage_profile	1
hold_newusers_posts	5
hold_guest_posts	0
attachment_limit	8
pickup_category	0
article_display	intro
send_emails	1
fallback_english	1
cache	1
cache_time	60

| Kunena integration settings:

Warning: Spoiler!

Kunena - AlphaUserPoints Disabled
Kunena - Community Builder 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:
en-GB	English (United Kingdom)

Third-party components: None

Third-party SEF components: None

Plugins: None

Modules: None

The first problem (the HTTP 401 "out-of-date bookmark" issue) is your Joomla menu. I don't know where you are redirecting your users after they login, but you need to change the location in the login module.

The second issue, that you are "unable to load user with ID: 97" indicates that user 97 is missing from your a database table. I don't know which database table is the one that's affected.

I'm not exactly sure what advice I can offer you under these circumstances. It may be better to revert to the backup you took before you upgraded and to restore your website to the state it was in before you upgraded. I would then use a test site (a copy of your live site) to go through the Kunena upgrade process again and verify if the same problems again occur. This seems to be an isolated case. It's my guess that there were database integrity problems that existed before you upgraded.