Forbidden 403 while posting

[rawonam]

When trying to post a reply or a new topic I (and all my users) get the following error:

Forbidden
You don't have permission to access /index.php on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

The url is the like this:
http://lingvoforum.net/index.php?action=post2;start=0;board=17

If I enter this url I do get to a posting page (with errors of empty subject and content).
Posting from Quick Reply works!
Everything worked fine yesterday night, a few hours ago, but something went wrong. I didn't touch any files lately, just created some groups and set permissions.
Any ideas? 

[rawonam]

I've just tried a clean installation without any modification to the files (though with the same DB), I'm getting the same error.

[rawonam]

Addition: when trying to change anything in user's profile, the same error is shown on saving.

[rawonam]

The hoster's support just restarted Apache and that solved the problem.