It appears you have an old version of PI. The current version installs 20140417-refdoc.zip. I recommend updating to the latest version of PI (reported on the console, latest version is PixInsight Core 01.08.02.1098 Ripley (x64) )
I dont know if it is related to this one. If yes, try the workaround there. If no, try rebooting your system. If this does not help, provide more detail, such as exact PI version, the OS you have, exact sequence of actions, etc..
Georg