Question sudden 500 error!

Hello,
this morning i found my whole forum gives a 500 error; the rest of the site is OK.
I haven't changed anything during the last weeks (months?).
No clue on what happened.
Any suggestion? Here are my settings:

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

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

This message contains confidential information

htaccess: Exists | PHP environment: Max execution time: 120 seconds | Max execution memory: 64M | Max file upload: 25M

Joomla default template details : psiconet04 | author: Maurizio Brasini | version: 1.0 | creationdate: 2011-02-23

Kunena default template details : Argent(RC3 - stable) | author: Gonzaunit | version: 1.6.0 | creationdate: 2010-08-20

Kunena version detailled: Installed version: 1.6.3 | Build: 4344 | Version name: Parlare | Kunena detailled configuration:

Warning: Spoiler!

[th]Kunena config settings:[/th]

board_title	Domande e Risposte
board_offline	0
board_ofset	0
offline_message	<h2> Il forum è momentaneamente inattivo. <div>A presto!</div> </h2>
enablerss	1
enablepdf	1
threads_per_page	20
messages_per_page	6
messages_per_page_search	15
showhistory	1
historylimit	6
shownew	1
jmambot	1
disemoticons	0
template	Argentrc
showannouncement	1
avataroncat	1
catimagepath	category_images/
showchildcaticon	1
annmodid	64
rtewidth	300
rteheight	300
enableforumjump	1
reportmsg	1
username	0
askemail	0
showemail	0
showuserstats	1
showkarma	1
useredit	1
useredittime	0
useredittimegrace	600
editmarkup	1
allowsubscriptions	1
subscriptionschecked	1
allowfavorites	1
maxsubject	100
maxsig	100
regonly	0
changename	0
pubwrite	1
floodprotection	0
mailmod	1
mailadmin	1
captcha	1
mailfull	1
allowavatar	1
allowavatarupload	1
allowavatargallery	1
avatarquality	75
avatarsize	2000
allowimageupload	0
allowimageregupload	1
imageheight	800
imagewidth	800
imagesize	150
allowfileupload	0
allowfileregupload	1
filetypes	zip,txt,doc,gz,tgz
filesize	120
showranking	1
rankimages	1
avatar_src
fb_profile
pm_component
userlist_rows	30
userlist_online	1
userlist_avatar	1
userlist_name	1
userlist_username	1
userlist_posts	1
userlist_karma	1
userlist_email	1
userlist_usertype	1
userlist_joindate	1
userlist_lastvisitdate	1
userlist_userhits	1
latestcategory	0
showstats	1
showwhoisonline	1
showgenstats	1
showpopuserstats	1
popusercount	5
showpopsubjectstats	1
popsubjectcount	5
usernamechange	0
rules_infb	0
rules_cid	205
help_infb	0
help_cid	1
showspoilertag	1
showvideotag	1
showebaytag	1
trimlongurls	1
trimlongurlsfront	40
trimlongurlsback	20
autoembedyoutube	1
autoembedebay	1
ebaylanguagecode	en-us
fbsessiontimeout	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
fbdefaultpage	categories
default_sort	asc
alphauserpointsnumchars	100
sef	1
sefcats	0
sefutf8	0
showimgforguest	1
showfileforguest	1
pollnboptions	4
pollallowvoteone	1
pollenabled	1
poppollscount	5
showpoppollstats	1
polltimebtvotes	00:05:00
pollnbvotesbyuser	100
pollresultsuserslist	1
maxpersotext	100
ordering_system	replyid
post_dateformat	none
post_dateformat_hover	none
hide_ip	1
js_actstr_integration	0
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
integration_access	communitybuilder
integration_login	communitybuilder
integration_avatar	communitybuilder
integration_profile	communitybuilder
integration_private	communitybuilder
integration_activity	communitybuilder
boxghostmessage	0
userdeletetmessage	0
latestcategory_in	1
topicicons	1
onlineusers	1
debug	1
catsautosubscribed	0
showbannedreason	1
version_check	1
showthankyou	1
showpopthankyoustats	1
popthankscount	5
mod_see_deleted	0
bbcode_img_secure	text
listcat_show_moderators	1
lightbox	1
activity_limit	0
show_list_time	720
show_session_type	0
show_session_starttime	0
userlist_allowed	0

Third-party components: AlphaUserPoints: Disabled or not installed | CommunityBuilder: Installed (Version : 1.3.1) | Jomsocial: Disabled or not installed | UddeIm: Disabled or not installed

Third-party SEF components: sh404sef: Disabled or not installed | ARTIO JoomSEF: Installed (Version : 3.7.6) | AceSEF: Disabled or not installed

Plugins: System - Mootools12: Disabled | System - Mootools Upgrade: Enabled | JFirePHP: Disabled or not installed | Kunena Discuss: Enabled (Version : 1.6.2) | Kunena Search: Enabled (Version : 1.6.2) | My Kunena Forum Menu: Disabled or not installed | My Kunena Forum Posts: Disabled or not installed

Modules: Kunena Latest: Enabled (Version : 1.6.2) | Kunena Stats: Disabled or not installed | Kunena Login: Disabled or not installed

thanks
Mabras

Your versions of Kunena component, Kunena Discuss and Kunena Search plugins and Kunena Latest module are unsupported. You are also using an Kunena template that was not developed, endorsed, maintained or supported by the Kunena project. You should upgrade to J! 1.5.26. CommunityBuilder 1.3.1 is out-of-date and is not supported.

Many people have reported problems using Artio JoomSEF. If you find that this extension is not working for you, then you should contact the developer of that product.

Please read HTTP 500 Internal Server Error for advice of what you can do to diagnose and fix your current problems.

You should upgrade your out-of-date software. You wrote that you haven't changed anything for months; I can believe that. K 1.6.3 was superseded in May last year.