Why can I not change my mailadress to one that I used before in a long time since obsolete profile?

Because you are only supposed to have one account here. Your 'obsolete' profile cannot be deleted and remains active. An email address can only be associated with one account.

Probably the best thing would be to change the email on the 'obsolete' account and then change the email on the account you are using now to the email you want to use.

As advised by @C-J-Horse, login to the old account and scramble the email address.

If you send a feedback though we can properly mark the address as deleted.