This is the error:
Warning [Xoops]: Smarty error: unable to read resource: "db:system_block_mainmenu.html" in file class/smarty/Smarty.class.php line 1084
Googling this error I notice that more people have the same problem, usually fixed by reinstalling apache and xoops, which is not an option for me. Could anybody give me a possible solution?
I have checked everything, the templates_c is writeable, and the system_block_mainmenu.html is there.
The error popped up when I installed teh Forum Module 2.2, if that is any help.
I am running:
Xoops v. 2.2.3
php 4.3.11

I get similiar errors once in a while. Try updating the forum module and system module. Then go to admin > tools > templates and see if you have templates that need re-generating.

That should do it, works for me.