And no, nobody else could know my username/password to that.
There's your problem.
The point of mining is to make the server more secure and secure from hackers taking bitcoins from people like they did for you. But now Bitcoin is so much safer due to so many miners. So you won't run into this problem again with bitcoin being as safe as is now.
This has absolutely nothing to do with the fact OP had his money stolen from an online wallet.