Problems changing registered e-mail address

Hi,
I would need some help. I have a Plex account, but the associated e-mail will deleted by the end of this month.
I wanted to change my e-mail address in the settings in my Plex account, but it will be refused. The Plex system says, this e-mail is already in use (Username has already been taken), but it isn’t.
This new e-mail address was used by mistake for another Plex account for me, which I deleted last week.
What can I do?

contact https://www.plex.tv/contact/?option=plex-pass-billing