Recently we got the below error while trying to assign the record.
Unable to save. This form can’t be saved due to a custom setting.

Turned out we had certain conditions in the OnSave event for the form, and if the record satisfies those conditions we were canceling the save event using
executionContext.getEventArgs().preventDefault();

So it was the expected behaviour, just that the error message was a bit generic.
Hope it helps..
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