Related to what @Will asked above, I have seen this week a few errors popping up regarding Invalid reference key on things that have been working for a long time., with no changes in the code. The only difference is that the account where I have seen this error is in 21.1 release. Is this also your case, Will? I believe it was happening in Client Scripts, where the field was set programmatically and upon saving the record. Also, some of the errors were solved if the field was shown in the form. But sometimes that did not fix it. And the ID was for sure correct, so it was not an invalid key for a given field. Anyone else experienced this? Any solution/suggestion? I did not open a support case for it (at least yet)

Yeah, it looks like we're running 2021.1 too.

you get the same advice as will did

In one of the cases, it was indeed restricted and that's why it could not selected, but in the rest, that was not the case. And it was really pointing out to the new release, since there was no changes to the code or the setup. I did not play around with some of the parameters you have when you set the value, maybe that would have help. I also made sure it was being executed as 2.0, as originally tested. I endedup just not setting those fields because they were not important. But yeah, strange. I would try to deep dive more if I face that again.