Mac shared as a Time Machine backup destination

This question has been asked but I do not find an answer, so I ask again.

I have an existing time machine backup disk on an iMac. I've purchased a Mac Mini and I would like to use the disk on the iMac as Time Machine backup disk for the new Mini.

I followed the instructions here, which look straightforward:

Backup disks you can use with Time Machine - Apple Support

On the Mac Mini I can select the drive on the iMac and I'm asked for user credentials. Once given, I receive the message "You do not have the necessary read, write and append privileges on the selected network backup disk. Please connect as a different user or contact your network administrator."

The user I'm using is an administrator and I've added it explicitely on the iMac with read and write access on the shared backup drive.

What can be the problem here? What is preventing the remote time machine to use the drive on the iMac, even if I followed the official Apple instructions step by step?

Any hint is welcome.