Strange Profile Problem

First time poster, but long-time user of portable apps!

I'm having a strange problem with Firefox. It seems to function correctly on most PCs but there's one that consistently gives me a "profile missing" error message. I click OK and then FF seems to function okay, although it periodically throws up the error.

I've error checked the drive, and tested in different USB ports, but the behaviour persists ... but only on that particular PC.

Does anyone have any idea what could be causing the anomaly?