Mail wants to make changes. Enter an administrator's name and password to allow this. Mail keeps asking this from within an admin account

Mail keeps asking multiple times for an admin account, after restart of the Macbook Air, and after restart of the Mail app, and sometimes just sporadically while the Mail app is running.

This happens since I bought a new MacBook Air and restored it from a TimeCapsule Backup (of an earlier MacBook Pro).

I read here that I should delete some certificates in keychain, but I do not know which ones. I do not find specific mail certificates.

Further, and funnily, the Account credentials I enter in the popup are the same as for the account currently logged in, as this is already an Admin account.

How can I solve this issue?