A member posted that he went to Mbank branches and call them several times and the final information Mbank gave him was that the cards could only be used by polish citizens and that was the reason why the cards were blocked. Later it seems he deleted the post where he wrote that, but he still states more or less that here (https://bitcointalk.org/index.php?topic=775730.msg8845280#msg8845280)

That reason never made any sense, taking in account that Mbank website uses several languages, I didn't read that in their agreement and that violated EU Law against european citizens. Also it doesn't explain why many foreigners still have working cards.

Even foreigners that went to a branch on Poland didn't receive their money back (https://bitcointalk.org/index.php?topic=746065.msg8905377#msg8905377)

It would be great to know the exact reason, because if the citizenship of the card owner isn't the reason and the blocks were caused by specific problems with the seller or owner of the cards, holders of active mbank cards could still use them (with small amounts).

On the issue of the transactions, anyone wanting to talk anonymously can use https://crypto.cat . However, I can understand why people won't be happy to give all details about their transactions. It shows too much about residence, habits, etc.

Besides, since they blocked cards massively, it won't be easy to figure out the reason by checking the transactions. The problem might be on the seller of hundreds of cards, not the owners; the problem might be on 1 of the 30 cards blocked, etc. It would be easier to ask again Mbank.