Time Machine backup to different subnet fails after update to macOS 10.13
There is a Lacie Raid attached to the Mac Pro. The Raid runs on HFS+, which doesn't get changed to APFS when updating the system to macOS 10.13 High Sierra. However, it seems that backing up onto a volume with HFS+ file system is not supported anymore.

I created another share on the system disk, which is APFS by now, and was able to back up onto this disk. Conclusion: Time Machine targets have to reside on APFS.