So, this wasn’t the case that someone wins a big amount of money and asks for a withdrawal. I must say that at that time I had lost about 400 Euros from betting at William Hill and I had about 350 Euros in my account.
When I tried to find out what had happened (via chat), I was told that my account is not fully verified. After about 40 days, my account was locked. I provided an online bank statement as proof of address and the account was soon verified. I opened a William Hill account about 3 months before.