Apparently the PHP editor I use introduced what appears to be automatically generated and harmless documentation comments that turn out to trigger a PHP error for some versions. The code looks totally legal, and doesn’t bother some PHP versions, but causes runtime errors in other versions. This causes error messages to show while logged in to the admin side. I suspect this is an actual bug in some versions of PHP. Never encountered this before, but a very simple edit seems to fix the problem.
Very frustrating, and I apologize.
The error no long appears on when tested on a site that show the error message. V 5.0.6 does not trigger that error.