I can't access the site, I get an error message, but as far as I can see it's the same requirement that the banks make. It's like WTF that they won't serve you funds won at another casino. I guess it's all AML policy. But come on, if I have to provide all the documentation to be able to bet at Stake, they're not going to see me there, with all the options there are where you can deposit, bet and that's it.
quoting what is written on the link: provided by @OP in case some have problem accessing the site
Acceptable documents for source of funds
Written by Marko
Updated over a week ago
Please note that the compliance team reserves the rights to request any document they might need.
You can use one of the following documents to verify your source of funds:
Salary ( You will need to send us a recent bank statement indicating receipt of your salary or original of your payslip )
Sales of shared or other investments / liquidation of investment portfolio / bonds ( You will need to send us a bank statement clearly showing receipt of funds and investment company name or interest on bonds received )
Sale of property ( You will need to send us a copy of the contract of sale and a bank statement showing the proceeds of the sale )
Inheritance ( You will need to send us a copy of the deceased's will, and a bank statement showing the received funds, along with your current balance )
Company sale ( You will need to send us a letter detailing company sale signed by a licensed solicitor or regulated accountant on letter-headed paper )
A Gift from a family member or close relative ( You will need to send us a donor's letter explaining the reason for the gift and the source of their wealth. Also, we will require certified identification documents from donor )
Winning the Lottery ( You will need to send us a receipt of winnings and documentation e.g. bank statement showing the origin of the funds initially deposited )
Divorce settlement ( You will need to send us a divorce settlement agreement and bank statement clearly showing receipt of funds )
Loan ( You will need to send us an executed copy of the loan agreement and a bank statement showing the received funds along with your current balance )
Mining ( You will need to send us a ledger of the amount of cryptocurrency mined, as well as a wallet address and transaction hash to which the mined cryptocurrency was delivered )
Your source of funds document must meet the following requirements:
Please upload an original photo or a PDF file of the documents, screenshots will not be accepted
All documents must be issued within the last 6 months and need to be in Latin international format
Shows your full name and address
Shows the name of issuing entity and date of issue
The whole sheet of paper is visible in the photo
winning funds from another casino is not acceptable
I can't find anywhere on the written requirements that funds from another casino is not accepted. I think this fall under the variation of lottery winnings where you need to send a copy of the source of fund you used in betting or wagering before you win the amount.
I believe there are variations that fall under those categories. I believe the casino is flexible in verifying your source of fund as long as you can establish where your fund is coming from. Like for example, if you are a part of a signature campaign, it is under the salary category wherein you need to present documents that shows that your fund comes from advertising a certain casino or company.
(It just pays for my other expenses, so that I can use my extra income for some gambling) 
