OP have you tried to summit to Stake proof of your positions and movements on the exchange you trade on?
That should be enough to demonstrate you earned your money through trading in a legal platform which also have your KYC. You could even ask the exchange costumer service to provide a document which states your lawful profits obtained by trading, this document could even include your KYC information which should match with the information held by Stake, in my opinion, that should be enough to convince Stake you are not doing anything ilegal.
Also, this topic makes me wonder: could Bitcoin earned by partaking in Campaigns on the forum lead to the same problems?
Would the spreadsheets issued by campaign managers be enough to prove origin of funds? Or would be necessary a document issued by the casino behind campaign itself to provide such proof of participation?
Sure, he probably could try that, but the question is: why does it have to come to this? Why should he be forced by a platform to show how he traded and where? It's not their job to demand him to show the whole journey the coins in his possession went through. He doesn't have to do that and he still should be able to request the return of his money.
It is possible that stake saw problems on the fund. I believe OP accounts is temporarily suspended, and will be unlock if he submit the needed requirement. It is too early to say that the casino is stealing the person's fund since we do not know the course of action of stake if @OP failed the needed KYC.
Some businesses have a rule in place that if you don't verify yourself you're unable to withdraw profits earned on the platform, but you are and should be able to (at least according to the EU law) get your own deposit back.
This should be the end scenario if ever @OP failed to the KYC procedure on source of fund.
We don't know everything, but I haven't seen any accusations towards OP from Stake about his coins being stolen or tainted like some people suspected. Sure, I'd agree with them if the coins were from a recent hack, or OP was suspected to be a scammer who made another account on the platform he once stole from. So far it looks like he did not submit documents required for KYC and they instead of preventing him from playing, blocked his withdrawals.
I think staked has sent a message to the OP that they are suspecting OP's account of possible money laundering, that is why they are asking of the source of fund.