Are you using any RX GPUs? For years I had issues where they would mine fine for a few days, then all of a sudden some GPU starts getting rejected shares. It would keep going up to like 10% rejected and the only way to fix was to reboot the rig. Usually fixed the issue. No idea why it happened because it would mine for days without rejected shares and then all of a sudden get rejected shares. I tried reducing memory clock or increasing voltages but it didn't seem to help.
So just keep rebooting and should fix the issue. If its a problem with the pool then obviously its not your GPUs fault and you just need to switch pools or wait for them to get it fixed. My advice is just use a miner like PhoenixMiner and use a pool like ethermine. There are guides on how you can easily get paid directly. Nicehash is great but there are issues from time to time.
This issue has nothing to do with memory clock or core voltage, I'm sure it's the pool that sucks, even if temp is too high it won't affect shares unless you overclock too high, the solution is to switch to another mining pool