I can reproduce the issue on Mac OS X Lion. It only happens with Sorter docked to the right side of the screen. So, as a workaround, just keep it on the left side.
To fix Sorter, so you can use it again, quit Sorter via the preferences pane in DevonThink Pro.
~/Library/Preferences/com.devon-technologies.think-sorter.plist in an appropriate
.plist editor such as Xcode, and delete the entries
NSWindow Frame Default.sorter and
Now, start Sorter again from the same preferences pane in DTP.
If you don't have a program that can edit binary
.plist files, you can also simply delete it. You'll probably lose your full screen preferences (Hide Sorter in Full Screen) though.