Solutions
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BambooForestServer's post in how to repair Parity after server update 7.3.1 was marked as the answerI found a solution by disconnecting the parity disk from parity one and reconnecting it to parity two, then running the parity check, stopping the parity check again, reconnecting it to parity one, starting it up again and running the parity check, and doing all of that in safe mode, and that worked.
Hopefully this works for other people too.