It turns out that even bitcoins coming from an exchange can be flagged as dirty. Yesterday, my wife sent bitcoins from her account on kraken.com to my account on bitpapa.com. My Bitpapa account was blocked
Nothing strange there, this can always happen on most centralized services and exchanges.
There are many reports of customers account getting frozen without explanation, and they can make up any story about this.
Even the tools they are using to say if specific bitcoin is ''clean'' or not is full of flaws and it was never known to be accurate.
You are lucky that coins will be returned to sender's wallet address.