3) Has there been a data leak on the BTT website? At least some years ago there was a warning on the page about "some hacking". That was the reason, why I changed my email address in that time. But, I do not know, if the email addresses could have been leaked then?
There was a user data breach in 2015 - part of it was user email addresses - so likely what happened was somebody figured out that your email had expired and re-registered it. I see old accounts coming back to the forum all the time; many of them were probably obtained through this same method.
Just wish campaign managers would do 5 seconds of research before accepting them into their campaign
by doing so they are only encouraging this kind of behavior.