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.

Then open ~/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 gridFrame
.

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.