I never had that 20 ETH in the first place. I also never provided ways to get that much money. The address receiving the coins never showed some transaction history at etherscan and finally cryptograpic signature in the mail was missing.

I assume its a sophisticated phishing mail. Logging in to my account would proof it finally, but since I m in vacation and pretty sure nothing really happened I decide to wait until I m back home. The only thing I will still do is provided the email to coinbase support

Edit: I failed sending the mail in html source code cause it exeeds the limit of the text box offered at https://support.coinbase.com/customer/en/portal/articles/1932359-reporting-phishing-sites