I have taken an environment from UPD9 to UPD21 two weeks ago and did not encounter any noticeable issues. There were of course some modifications that required adjustment but nothing out of the ordinary and no issues with the Core portion of the UPD.
What sort of errors? Not that we have / are planning to deploy UPD21, just out of curiosity. Probably these are addressed somewhere in the community one by one.
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