when they revealed the unhashed server seed I checked it using a SHA512 calculator... It should match the hashed server seed they provided to me, however, it does not match.
So the hashed SHA512 of the server seed is different from the hashed server seed they gave to me when I started my session.
First, I would suggest you share the unhashed server seed here so everyone can double-check it. Did you calculate using binary hash or string hash? Sometimes it was the problem.
https://bitcoin.stackexchange.com/questions/40924/op-sha256-mismatchYes indeed, free online tools could have discrepancies in result. You should first double check. Maybe, you're just doing something wrong in your process that the output is different from what you've expected.
Better post the hashed server code here, so the community can help or guide you, on how to throughly check it.