APFS Volume on ext. Disk not mountable - space manager is invalid

I was using a APFS external USB Disk on a PC using a commercial driver. Suddenly it stopped being accessible.. Now I can´t mount the Volume inside the APFS container anymore, on PC but also not on a Mac! If I try diskutils - please see the log:

123$ diskutil verifyvolume disk4s1
Started file system verification on disk4s1 DISK-XYZ
Verifying file system
Volume is already unmounted
Performing fsck_apfs -n -x /dev/rdisk4s1
Checking the container superblock
Checking the space manager
error: (oid 0x400) sm: invalid o_cksum (0x74dfcb428a5691c4)
error: verification/reading of the spaceman object failed: Illegal byte sequence
Space manager is invalid
The volume /dev/rdisk4s1 could not be verified completely
File system check exit code is 8
Restoring the original state found as unmounted
Error: -69845: File system verify or repair failed
Underlying error: 8

Any idea how to fix it? As there is important stuff on the drive I currently draw an image to be held safely while testing on the drive. Or should I try the other way round? How do you evaluate chances to fix it with fsck? Do you know of any other handy tools? Disk Warrior does not handle APFS unfortunately.. ah- btw - mounting read only does not work unfortunately.

123$ diskutil mount readOnly /dev/disk4s1
Volume on disk4s1 failed to mount; it appears to be an APFS Volume which might be locked
Try "diskutil apfs unlockVolume"

Thanks and best, DHVD