Not possible to update UPM over the "Update now!" link under "Manage My Plugins" in Jira 5.0.5

Hello,
after updating from Jira 4.4.5 to 5.0.5 it is not possible to update the Universal Plugin Manager (UPM) via the prompt for update under "Manage My Plugins" in the administration interface. When clicking on the link "Update now!" nothing is happening. It looks like the Browser doesn't even try to load anything, so it's maybe AJAX related? We've tried IE9 and Firefox 7.0.1

We recently updated our test environment from 4.4.5 to 5.0.4 (5.0.5 wasn't available then). It's nearly exactly the same setup(except from the data within it). There this link to update the UPM works in IE9 without the problem.

Thank you very much!

Br,
Manuel