We don’t use Legal number maintenance, so I’m just throwing things out there …
(also, I’m assuming that 10.2.500 is still being tested, and not yet being used for production)
Did you verify that the settings on the Detail tab (on 10.2.500) match those from 10.2.200? Maybe they newer version has an extra field or two, or that somethings didn’t come over in the migrations.
Have you created Fiscal calendar/periods for 2020 in the 10.2.500?
You said it works when you let it choose the sequence. So it might have already created legal numbers for this sequence. And the number you are selecting as that start might conflict with existing sequence numbers used.
Once again, I’m just tossing around things to look at.