Recently we have moved our Invision Power board (version 2.3.3) from InvisionPower hosting to a dedicated server. On our new server we have:
Apache 2.2.6
PHP 5.2.4
MySQL 5.0.24

Things seem to be about 95% OK, however, there are occasional problems with posting: several members tried to post Hungarian and French characters, like

These are not getting through, and they get an error:

Bad Request

Your browser sent a request that this server could not understand.
Apache/2.2.0 (Fedora) Server at Port 80

Members have been asked to try with Explorer, Firefox and Opera, and all get the same results. This is strange, as most Croatian and Serbian characters that are accentuated, like:
č, , ć,

These go through just fine, as well as Cyrillic alphabet is OK as well.

Additionaly, one member reported a problem that 3-4 times he got an error while posting (but is usually OK to post), and he writes in Serbian Cyrillic - which seems to be usually fine, but there is an odd problem and error message:

Method Not Implemented

POST to /forums/index.php not supported.
Apache/2.2.0 (Fedora) Server at Port 80

We asked Invision Power Board tech support, however they say that the errors are on the server end in the Apache configuration, and not IPB. Which seems logical, as before the move on the new server (I don't think the old server used to run Apache), nothing like this used to happen.

We'd appreciate any suggestions.