Thanks, Terence. Rebuilding the library solved several problems, including the export failure.
I had used Mac disk utilities to repair permissions earlier, but it found not errors and made no difference.
Because I was still having some problems (missing events in the event view), I tried rebuilding all thumbnails (est. time 2 hours). About 2 hours in, it failed with no error message (just "report to Apple" pop up). [I reported the failure to Apple.] I rebuild the known missing event by flagging the photos in the photos view and "creating an event from flagged photos".
Subsequently, I noticed that the metadata (image dimens, size, rating, tags, etc.), that had been shown at the bottom of the left column, is now missing. I haven't been able to find an option to view them or not so I'm guessing they were lost in one of the library rebuilds...?
So am I doing something stupid?
Regards,
RL