No issues here. Try disabling your cookie manager extensions completely to see whether they are causing you the trouble at all. There might be something else doing this. You seem to be very privacy-minded; are you using VPN by any chance?
Naah, I'm not going to disable cookie manager/cleaners just because one site can't authenticate users properly - I'll simply just switch to another site/pool.
No VPN at the moment.
Well, don't blame the pool for what you are experiencing when no one else is having this issue. The problem relies in your machine clearly. What difference does it make adding an exception or turning the extension off completely when using MPH? Especially for testing...