When I click on write a comment I get the following warning displayed:

Warning: call_user_func(JOSC_ubbcode::convertlinebreaks) [function.call-user-func]: Unable to call JOSC_ubbcode::convertlinebreaks(xyz) in /var/www/html/website/components/com_comment/classes/ubbcode/stringparser.class.php on line 381

where xyz is the content of the comments.

I have read in another topic the advice to ask your host to upgrade to php 5.2.

That could be the cause as my host is still on php 5.1 (check http://www.f6clan.com/website/sysinfo.php voor phpinfo ).

But I didn't find any requirements about php 5.2, only that it works with php 5.

Is there any workaround for php 5.1 ?

You can check the warning at http://www.f6clan.com/website ... just click on write a comment below the first article. Only registered users can write comments, but you get the warning for the existing comment.