The problem lies in the fact that from September the 1st, HKLM\SOFTWARE\Microsoft\Microsoft Antimalware\EndOfLifeState key changed from 0x2 (Grace period. OS has reached end-of-life, but anti-malware platform service is still running and definition updates can be received) to 0x3 (Anti-malware service stopped. You can no longer start the anti-malware service). As suspected by heinoganda, replacing the gui (msseces.exe) is no longer sufficient.
@heinoganda
Any chance to try your last modded MSE?