Found the problem: My password contained special characters, which did cause the problem. I haven't tested, which specific character it was, but I think it was either \ or | as well as the password doesn't cointain " (just tested, I don't mind posting the characters, since the password was auto generated, there are many wallets out there which are easier to crack than mine).
@Devs: maybe you should look into this problem. As said, I got no problem withdrawing any coins, just the unlock in the console wasn't possible.

I just changed the password so I am able to unlock the wallet now.