Since upgrading to Mojave, my Macbook's time machine has refused to backup to the Samba machine. I wasn't sure if it was something they'd done to try convince me to buy a Time Capsule (as if!), or something that went wrong in the upgrade process... so during my time off, I decided to reinstall my Mac and see if that helps.
It did not. :(
Digging in to the inner workings of time machine, it seems that you can browse the time-machine specific logs here (they don't seem to be logged to a plain-text file, looks like everything is going the way systemd is!):
log show --predicate 'subsystem == "com.apple.TimeMachine"' --info
I neglected to save the actual error message, but it was basically a "file not found" error trying to create a file under
/Volumes/com.apple.TimeMachine.timemachine/fwaggle-mbp/, so on a whim I tried:
and surprisingly, it's working now!