Adding cross references.
jarmo in GitHub issue #3315 got this on 2.0.3.9 seemingly right after upgrading 2.0.3.5.
ts678 updated #4286 with two examples of “Unique constraint” (and lots of other errors).
There seem to be some patterns in the errors there, but expert interpretation is needed.
If anyone else has found “UNIQUE constraint” in a string of errors, the details might help.