I've been having success lately with English language sites where flex vs flex regardless of deposit/prepayment has been approved quickly with no issues. I don't think that memo has been sent to all associates reviewing non-english language claims though. The reply I received this morning was:
"You have booked a rate that is fully flexible without any deposit. The competing website, [xxx] does only offer prepaid rates. Since the rates are not comparable, I am not able to approve your claim." It's clearly stated on the OTA that "Cancellation: Full refund will be possible until Wednesday, 09/19/2018, 12 o'clock (CET)."