Are you playing fullscreen? I\'ve noticed that the error dialogs don\'t agree with PlaneShift when its fullscreen, they tend to make the screen go black. (I assume your using windows?)
Next time it happens press once, wait a few seconds then press it again. It should drop you back to desktop and create a Diagnostic (*.dmp) file in the main PlaneShift directory.
Theres a thread where you can post the dmp file so they can fix the bug.
Oh, and in future please try to post your system specs along with your problem.