it turns out this must have been caused by a staralignment process icon on the desktop i was trying to save. after deleting them all, i was able to save the project. i have the .psm file from /tmp which should include the bad process icon, but unfortunately i did not attempt to save any of the icons separately, to see what was wrong with them.
clearing and closing the staralignment process itself did not help the problem. the reference images in all the icons was a disk file.
rob