A few months ago I created a fresh yandex account and used the email address to register at a bitcoin trading platform. I subsequently bought some btc and kept them in the web wallet provided by the market. So far so good.
When I tried to login to my yandex account later I saw the message that they had seen some obscure "suspicious activity" regarding this account and therefore I could no longer access my yandex account. Effectively I was now locked out and also could no longer login to the bitcoin web wallet due to the "login guard" option, which requires me to verify each new login via email verification.
I was startled since I only used the yandex account for this very purpose, nothing else going on there. Neither did I login via tor or any kind of proxy which might trigger such a response. The bitcoin trade was legit too. At that time I couldn't figure it out and concluded I must have made a mistake somewhere and no longer thought about it.
Now I was stupid enough to do the same thing a few month later. The exact same thing happened. My bitcoins again gone.
My system is 100% not compromised since I'm setting up fresh Linux distros constantly and always verify my images via PGP.
Conclusion: someone inside Yandex is selectively stealing bitcoin.
My advice: Stay away from Yandex if you're interested in keeping your bitcoins.
I'm now using another email service and the problem didn't reappear.
Its sad since I kinda liked them.