Change in password and email have indication that even account have been compromised. Usually users will reason the same as you when asked to sign a message to prove account ownership of one of the bitcoin addresses they've used, they argue they don't have access to a wallet because bla bla bla. That's a common reason why someone assumes you are not the original owner of this account.

I have a list of bitcoin addresses you used previously here before email and passwords changed, if you can sign a message from one of them then maybe we can think of something different about you.