For my RX 580 8Gb rigs Phoenix 5.1c didn't create DAG epoch №385. It creates DAG for 0,5 sec and all shares are incorrect. Speed shows normal.
I think PM just didn't support epochs 385+
Looks similar here, since estimated 6 hours almost all shares on
ETC-Mining (Ethermine) are invalid.
ETH seems to be working properly, still ...
Doublecheck with NBMiner, no issues! Seems to be a problem with Phoenix only .... oh wait, NBMiner produce many (>20%) rejected shares at ETC-Mining currently ...

I'm use lolMiner now. All fine.