Are they saying that it is an entity external to Openchange that has freezed the funds?
That's exactly what it sounds like. See their other comments:
Funds from the client were not received by us, so the order cannot be completed.
We do not perform AML and KYC verification, we only request supporting documents upon the request of the exchange, about which we inform the client.
All you need is simple verification. What's the problem? Make photo and screenshots, that's all. After some time funds will be credited to our account and we will make refund.
Janyiah, we can pay the order only after coins deposited to wallet, as was mentioned before the coins are frozen due to AML checking at this moment
And this quote from their Terms of Service:
4.2 To receive Payins from Users, the Company uses the requisites of crypto exchanges. By sending a Payin, the User understands that he is not sending funds directly to the User's cold wallets, but to third-party accounts where the Company has accounts.
Sounds like OpenChange need to come clean here and tell OP which third party service is currently holding his coins ransom.
OP could also pursue this avenue:
7.6 The user has the right to receive the results of the audit and/or independently verify the correctness of the risk assessment.
I would be asking for the results of the audit so he can independently verify how they have decided that mixed coins are "100% stolen", since that is clearly and provably incorrect.