Tested this on two separate WP 2.7.1 installations:
When writing a new post or editing an old one tag suggestons fail if the user is of Author level or below.
When the user starts typing a tag the "suggest as you type" box appears but it displays only "-1" (minus the quotes), which is also addded to the tag field if clicked. The same "-1" appears when the user clicks on the "Choose from the most popular tags" link.
Tested with all plugins disabled, the bug persists.
Anyone having the same problem? Any help would be greatly appreciated. I'm at my wit's end here.