OK, so it looks like the problem has something to do with me including a '/' (slash) in the collection name. Devs should probably provide a warning message that certain characters are not allowed when creating collections.
I can't reproduce that misbehavior with a collection named "A / B". Many people are getting that error message when trying to add to the Quick Collection and other collections whose names don't include "/". LR 10 tried to fix a longstanding bug with custom sort order of collections and folders, and the "fix" introduce this new bug:
The workaround that I fould was to create a new collection then copy the contents of the old collection into the new one and delte the old collection. Annoying that it needed to be done, but at least it's a fairly easy "fix".
I was also getting this error. I couldn't use B or dragging a photo to a quick collection I created. I copied the photos into a new collection and deleted the old collection. That worked. There was nothing unusual about the name of my quick collection