There seems to be a conflict with Prolific's new authentication flow (or something). It appears are experiencing this issue even without Prolific Assistant (PA) installed but PA seems to make it worse. Prolific is working on a fix on their end (asap but its the holidays so it may be slower than that). I'm closely monitoring for any updates on this and will add any fixes on my end if required.

Edit: This issue has been fixed by the Prolific team.