And the lesson is...

…don't release software at 2am!. By that I mean, of course, that release 2.2a had a very minor bug.

The map display may have black blocks or bands when re-drawing after the completion of some calculations, on some systems (only Win2k so far). This occurs when the user-interface thread tries to draw the display buffer but is denied access to a map that is in use by the worker thread. Instead of aborting and flagging for a re-draw, the buffer drawing routine aborted without setting the redraw flag. The problem has been fixed, and the newer version is available on the downloads page.