The problem is that the deposited funds were converted to chips and then swept by
someone... You claim that it wasn't you, but that could be tricky to either prove or disprove

As LoyceV has suggested, the timestamps on the messages sent to you might be key to this... if the message was sent after the funds were swept, then it couldn't possibly be you. If it was sent before, then it's possible your mailbox has been compromised and someone else got the session key, created the chips and then swept the keys.
Yes that was exactly what I tried to explain to ChipMixer after they provided me a new session key where the chips were already sent. My mailbox was not compromised.
Hello
After investigation we have discovered that session awarded with chips was started 5 hours after deposit transaction. While destroying session ends our contract responsibility and funds were already used by other (lucky) user - we award you full value voucher for 178 mBTC.
This is what I call a RESPONSIBLE COMPANY!! Thank you for taking care of your customers!!
I will close this discussion now. LONG LIFE to CHIPMIXER!
Thanks a lot to everyone!!!!!