That happens when you "lost" your admin-session information, e.g. due to a timeout. Try signing into the admin-section in a second tab and reload the page in question.
If you can work other "admin" pages but not the plugin manager section, that would be a bug, I guess. Try downloading the manager manually and installing it via directory-placement and place a bug in confluence-jira.
Oh, and post the solution here :)
Good luck hunting
I have the same problem, I even changed browser and got the same problems after visiting the "Plugins" section in the Confluence Admin. The latest thing I did was trying to install "Timesheets plugin for JIRA" in JIRA, on the same server but not linked. If my memory is OK :(
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