I looked at the charts and transactions intensively and couldn't find any extraordinary transactions which may be related to an attack. Quite contrary, it looks like the pool wallet slowly starved out over the last two month until it was down to 0 BTC on the 17th.

Given the fact that the pool operators couldn't find any flaw in their billing system I think it could have been victim of a block solution withholding attack. On a PPS pool the damage of block solution withholding fully affects the pool owners, so a group of miners with enough hashing power (10% of the pool hashrate, so about 10 TH/s for 50BTC) would cause the pool to lose money over time, without having any loss themselves, as they get paid per share, even if they prevent block solutions to be sent to the pool.