Originally Posted by
randomnumber46
I DM'd you but not sure if it went through, so replying here as well to be safe.
The error only occurred with Polymorph, since it was the only shapechange effect with concentration. The effect applies correctly, but when you break concentration (either by manually deleting the effect, failing a concentration check or clicking the 'Stop Concentrating' button) the error pops up. It doesn't seem to happen if the polymorphed form is reduced to 0 hp and the creature changes back, even though this also removes the polymorph effect. The same thing happens whether the caster is the target or not.
Everything seems to work correctly, even with the error, but since I don't know what the error is I don't know if there is anything unseen going on so I've avoided using them together.
Let me know if you want me to go into more detail on any particular part!