Somehow forum's got associated to wrong taxonomy vocabulary. Need help
Option 1
Go to the Forum Topic manage fields and click edit in the forums field.
Check that the vocabulary is set to Forums
If it's not, delete the forum field and add the term reference field again.


Option 2
Have u posted content already to the forum? or do you care about the forum content? if not, disabling and uninstalling both Forum and Advanced Forum would be an option you could try and then enable them again.