It could be huge deposits that he can't justify from which source it came and the stake is looking for a legitimate source that's not related to crypto.
And if that's not acceptable to them, OP has got no choice but just to leave it since he has already asked and contested about it. There's no way to change that decision from them. You can still gamble with other casinos if you want to OP but, there might be some really huge thing as to why they've banned your account related to your deposits/withdrawals.
The thing is, OP is already at the level 3 and why there’s a sudden change?
Probably OP tried to deposit huge amount of money and that trigger the stake to investigate the account. Maybe Stake can do something about this, they can easily know if OP is a real gambler who already spend a lot of money on the platform. There’s no way for OP to let go of that money, the only option is to negotiate or comply with the requirements, show them your proof of trades and maybe that can work.
It seems that they can't do anything with it anymore and OP has to deal with their decision and that's it. In this case, negotiations can still do but if they've got placed and given the final decision already, then the affected gambler has to accept it lightly whatever the decision is.
There's really something that has triggered them with OPs, knowing that he's got that level already, there should be some benefits and privileges but stake can freely honor it or not.