Did you just finish a battle and we're trying to change classes before the server had fully registered the battle as complete? Naturally you can't switch classes while in combat so it's possible there might have been a bit of lag between your client and the server which resulted in the server still believing you were in battle for a few seconds more when in fact it had finished.

This happens to me all the time, particularly after just finishing a FATE, as I find other functions are often locked when in battle and don't always update straight away (such as being able to ride a mount). All you can really do is just wait a little bit for the server to 'catch up' and try again.