File sharing: password/user not recognized

I am trying to share files and access volumes between my iMac (2009, El Capitan) and my MacBook Pro (2018, Big Sur). Both user accounts have identical user names and passwords.

File sharing (and screen sharing) is on: SMB and AFP on the iMac, SMB on the MacBook.

Firewall is on: Allow incoming connections for File sharing and Screen sharing.

The Macbook can access the iMac no problem. From the iMac, however, I cannot connect - either the user name or the password (or both) are not accepted (shaking login window).

I tested with another specially created administrator account on the MacBook - no luck.

Screen sharing works both directions, though.

I had done a clean install of Big Sur, and all was fine with the user account that was created on installation. After migrating from a TimeMachine backup (with user Library and Network Settings) and deleting the installation account the above issues occurred. BTW, I had the same problem earlier, running High Sierra.

Any hints will be greatly appreciated!

Jörg