The problem seems to be related with the FeedWordPress Duplicate Post Filter plugin. Once you deactivate it, it starts to work again, apparently.
I have the same problem.
Upgraded to 2.8 and cannot add any tags to new posts but ONLY in two of three wordpress installations.
I’m trying to found out what is different in the one that works.
There’s is also other things that don’t work like drag and drop menus.
