I did an RDP session to a remote host. In fullscreen mode, the remote taskbar was hidden by my local computer taskbar.
How-I-fixed-it:
Bring up the task manager on the local computer, right-click "Windows Explorer" and select "Restart".
I re-opened the RDP session and now the Fullscreen worked fine again.
I found the solution here:
http://superuser.com/questions/495871/taskbar-disappears-over-remote-desktop-connection-when-outside-local-network