There are a few quests currently bugged at this moment, and sadly the class change quests are affected as well.
We've been told they might have found the reason but there's no set date for a fix. I'm pretty sure it will be fixed next time we go down for a maintenance (either the regular scheduled one or an emergency maintenance).
If you really don't want to wait, you can try sending a ticket to support.warpportal.com and request that they change your character's class. You'll have to supply evidence of the quest being done (your level + the gear in your bag should be enough) and follow their instructions if needed. You might need to be logged off during this procedure, though.
Now, while this is possible and I've been told they'll try to do it in response to the tickets received, keep in mind that it's possible to play the game without changing classes, and that with the number of players affected, it's possible you'll have to wait a bit anyways.
Whatever you choose to do, i can only ask you all to try and be patient.