It's possibly a php error that's not getting outputted. Go to Dashboard -> Sitewide Settings -> Debug, and make sure "Debug" mode is set (not "Production". Then if you go back to the Add Functionality page you should see the error (assuming that's the problem).
If that doesn't work, do you know which things you've added to /packages or /blocks recently (or downloaded from marketplace recently)? Try removing them one at a time and then re-testing the page to see if you can isolate the problem to a specific package or block.