I figured it out, but it's really bizarre. When I took out the image alt and title tags, the update went through.
All the other thumbnails I had added to the group page had alt and title tags though, so I have no idea why those
attributes kicked up errors for this particular thumb. This particular error has happened (very infrequently) before
though for random wall thumbnail adds.
FYI sheq the alt and title code that I took out was: alt="Days on Clouds" title="Days on Clouds" for
Days on Clouds by segnoj
(With the alt and title the thumbnail worked in preview mode, but kicks up the error message when I submitted the
change.)