Installing and upgrading help

This worked for me. That is:
Go to …/server/moodledata/.
You'll find (among others) two folders: cache and sessions.
Delete the contents of these two folders. (I didn't have to restart the browser, a simple refresh worked for me).

I'm just adding this answer to this old thread because this problem still occurs [I'm using Moodle 3.1 (the latest version at the time of writing this) on Windows 7].