I'm having the same problem, but I've seen it in both 2.2.2 and 2.2.3.

I had everything working just fine in 2.2.2. I had my share directory setup as the same directory as my save directory and it was working just great until one day I made the mistake of exiting Windows 2K with LimeWire running and minimized in the system tray (a seemingly normal operation).

When came back the next time on reboot most (not all--odd) of my preferences were lost and I was unable to set my save directory to the same directory as my share directory without getting an 'invalid directory for saving files. Please use another directory or revert to the default.' In fact I get this message no matter which directory I try.

I uninstalled 2.2.2 and reinstalled 2.2.3 and specified my save directory to be the same as my share directory during installation but it still defaulted to "C:\Program Files\LimeWire 2.2.3 Pro\Shared". I even went into the LimeWire.props file and edited the DIRECTORY_FOR_SAVING_FILES line and it ignores the line! Excellent.