I'm going to take a wild swing at it being either bad local backups or a corrupted TM drive - or both - as I've had nothing but trouble with both ever since APFS was introduced. Mojave has been better than either of the Sierras, but still it happens occasionally [across two different Mac Pros running 2 different OSes, both with a permanently mounted internal TM drive.]
I can usually force backups to start to behave by clearing down 'stuck' local backups from Terminal but frequently I've had to use DiskWarrior to also fix the Time Machine drive itself. Hopefully your TM drive is still HFS+, as DiskWarrior can't fix APFS yet.
To clear down local backups, open terminal & type
tmutil listlocalsnapshots /
That will give you a list, similar to
For some unknown reason it just seems to hang onto way more of these than seems sensible, though it should clean them up itself.
You can clean these up forcibly using
tmutil thinlocalsnapshots / 1000000000000 1
[this is an overkill number, designed to clean every last one of them, aggressively; otherwise you just seem to have to do it over again]
List snapshots again, as above & you ought to end up with just one. If that ends in
(dataless) then try a reboot & see if it goes away on its own. If not, you should be able to clear it manually using the date/timestamp, for example
tmutil deletelocalsnapshots 2019-02-10-163737
If you find one with an
pm timestamp, see APFS/High Sierra - tmutil deletelocalsnapshots will not delete snapshots - Error parsing argument
Once you done the tidying up, then let Time Machine try one more run. If it fails you may need to repeat the above. I'd be almost certain, though, that you will have to run DiskWarrior over the Time Machine drive & let it repair it. I have never found any other utility that can fix drive corruption like this. Disk Utility won't do it.
Note: I have no affiliation with DiskWarrior; merely a satisfied customer. It does seem to be able to fix things nothing else can touch, though I have no clue how it actually achieves this.