Have a look at the manual's page.

Could this problem be solved by deleting all the posts in my forum? I that case, how can i do that?

--------------- Added [DATE]1216034378[/DATE] at [TIME]1216034378[/TIME] ---------------

I got this error just now:

Database error in vBulletin 3.6.8:

Invalid SQL:

SELECT template
FROM vbulletin2_template
WHERE templateid = 1672;

MySQL Error : MySQL server has gone away
Error Number : 2006
Date : Monday, July 14th 2008 @ 01:07:05 PM
Script : http://www.fotballprat.net/newthread...ostthread&f=64
Referrer : http://www.fotballprat.net/newthread...newthread&f=64
IP Address : 89.8.144.13
Username : Mourinho
Classname : vB_Database

--------------- Added [DATE]1216038338[/DATE] at [TIME]1216038338[/TIME] ---------------

Can someone help?

If your going to delete your posts why don't you just do a fresh install?

That would solve the issue.

See the manual: Lost connection to MySQL server during query

PS Deleting posts or start with a fresh forum will most likely NOT solve this problem.

Hmm.. i see.... I don't know much about mysql, and I find it difficult to fix too :(

Got a new databaserror now:
Database error in vBulletin 3.6.8:

Invalid SQL:

SELECT post.postid, post.visible, post.userid
FROM vbulletin2_post AS post

WHERE post.threadid = 1803
AND post.visible IN (1
,2
,0
)

ORDER BY post.dateline;

MySQL Error : Lost connection to MySQL server during query
Error Number : 2013
Date : Monday, July 14th 2008 @ 02:59:37 PM
Script : http://www.fotballprat.net/showthread.php?p=24243
Referrer : http://www.fotballprat.net/newthread...newthread&f=66
IP Address : 89.8.144.13
Username : Mourinho
Classname : vB_Database

--------------- Added [DATE]1216040670[/DATE] at [TIME]1216040670[/TIME] ---------------

Is this something my webhost can fix for me?

Again, see the page Marco has linked to. You should also ask your host as well, say that you are getting "Lost connection to MySQL server during query" errors.