I seem to be having the same problem.
Logged in as a user with Administrator privileges I can edit or create a new post and click on "Choose from the most popular tags" and see all the tags that have been used and select accordingly.
When logged in as a user that has Author privileges, when I click on "Choose from the most popular tags" the only thing that shows up is a -1. I have changed the privileges for this account from Author to Administrator and the tags show up just fine, but as soon as I change the privileges back to Author the list changes back to -1.
I Have NextGEN Gallery plugin installed and I thought that may have changed something within my site since it does work with the tags. I have Subscribers selected for Tags under the Roles portion.
I am thinking it is a permissions issue; I'm just not sure where to check or what to change. Any help would be appreciated. Thanks.MJI