Mail.app archive button stopped working

A few days ago the Archive button in Mail.app stopped working. Has anyone else had this happen or have any idea how I might troubleshoot this?

A bit more detail:

• I have Mail.app set up with two accounts - one Gmail IMAP account and one other IMAP account. This problem happens with both of them.
• Highlighting a message in the message list and clicking the Archive button does nothing. Ditto with ctrl-clicking the message and choosing "Archive" from the context menu;
• HOWEVER, opening the message in its own window and clicking the Archive button from the message's toolbar works fine (I added it to that menu bar)
• This morning I completely reset Mail.app (deleted the /Library/Mail/V2 folder) and re-added my accounts. The Archive button functioned normally for a little while after that, but has since returned to being non-functional
• The only third-party Mail plugin I'm running is Mail Unread Menu (http://loganrockmore.com/MailUnreadMenu/). I've tried disabling that and it doesn't seem to make any difference.
• All the other toolbar buttons are working fine - delete, reply, etc.

I'm wondering if this is a problem in 10.7.2? Apologies if anyone else posted about this, but I haven't been able to find anything about it here or using Google.

My system:
Macbook 2.16, 2 GB RAM, Mac OS X 10.7.2, Mail.app version 5.1

Thank you for any help or ideas!

 

I should add that quitting and re-opening Mail has no effect, nor does restarting the computer.

 

Thanks, Coden! That worked for me, too. I had thought I'd tried disabling it, but I guess I didn't uninstall it completely.

Thanks again!

 

I can verify that the new version of Mail Unread Menu doesn't disrupt the Archive button. Thanks, Logan!