I have another data point; it doesn't appear to be related to scanning into the Catalog.
I scanned a NAS drive directory into a catalog and when it was ocmplete, I just terminated PM+ and walked away.
Some time later, I started PM+ and created several collections in the catalog.
I searched the catalog for images in the first collection and it returned around 900-1000 images.
As detailed above, I do a Ctrl-A, then right click to add images to the collection. The error seems to occur if the Crtl-A and add images to collection steps are intiated 'too quickly', i.e. if I slow the steps down then the error does not seem to happen.
I suspect, but do not yet know, that once the the search results ae returned, if I Crtl-End, and wait for the file names to change color, then Crtl-A and add to collection, the error might be avoided.
Again, the error is not fatal; in fact once it ocuurs, just count to 10 and try again and it will usually work.
Best Reagrds,