Serious question. How is it even possible that someone changed the mail address of the mentioned account?

It's absolutely impossible that the person had access to my email address. This is most likely a hack due to the leaked bitcointalk database.

So, how was the person able to change the email address?

Also, as said, there is no address linked to my account. I contacted the new account holder now and referred him to this thread. I'd like to know how he got the account. Probably bought it in the forum from someone who hacked my account.

Again, an admin would just need to set back the email to the address that was used previously.... then I could recover my account. Apart from that, admins could contact the user celeborne, he knows information's about me that the new account user can't know. Also taking a look at the IP's and posts that I mentioned should show that there is something wrong.