FDP cannot repair damaged disk resp. disk volume

Macbook Pro 2019, Intel, Tahoe:

When I run the FDP Help/Repair function on an container from an external physical disk, formatted in AFPS with app. 20 volumes, then it tells for one of the volumes:

"Das Volume „/dev/rdisk4s25“ mit der UUID 1B438AC1-3351-4818-AA0E-6F886F83BB51 ist offenbar beschädigt und muss repariert werden"

i.e. "The volume /dev/rdisk4s25 apparently is damaged and must be repaired"

The top line of this FDP run tells:

"First aid hasn't been successful. Please make a backup..."

Then I ran with terminal:

diskutil repairVolume /dev/disk3s1

This did run some time without error messages, but doing again FDP Help/Repair afterwards, on the whole container, yielded the same error as above.

I remember that I had a similar problem some months ago und found a thread here, where somebody suggested for the same problem a "sudo ..." command line.

This did work fine for me, but unfortunately I cannot remember its title for finding this threat again. Although I searched quite intensively on this forum.

Can anybody help either by referring to this thread or by providing his own guidance for a solution?

Thanks very much.