This sounds like bullshite!
If the error was with the PROMOTION, there would be no need to void ALL GAMEPLAY for that day. If this was a cashback, was this one on WAGERING, or on the amount won or lost. If on losses, there would be no real problem, as players would already have lost the money anyway. If on WINNINGS, the casino might find they had offered too much, and want to get out of the offer, but again, no need to void gameplay. If on WAGERING, it could be down to bad design, similar to the Big Dollar fiasco some while back. This could result on a guaranteed +EV for players who played games, even if they lost, because the cashback would put them into profit (this is what befell Big Dollar). If the problem was supposedly with the underlying games, then this is a SOFTWARE ISSUE, i.e, the software was "cheating", but in a way that favoured the PLAYER, and it would NOT be enough to pull the promotion, but that they would have to find a way to void winnings. This is a HUGE hit to their credibility, as we now view their software with suspicion, and wonder whether they would have been as proactive were the error to have favoured the CASINO (from past experience where this has been the case, I would say NO!).
There was a previous issue where they refused to pay a player by a convenient means (but one allowed in the T & C), and insisted they take a cheque, which was highly inconvenient, and would involve long delays and several layers of fees. This player only wanted their deposit back after being told they would NOT be paid according to the general provisions in the T & C, but that they were viewed as an "exceptional circumstance" which meant adverse treatment for their withdrawals.
I would say
PAB, but also tell us what this promotion entailed, so we can see if they are just plain daft enough to design another "Big Dollar" type +EV offer, or whether they are trying to cover up a software bug by pretending it is related to this specific promotion.