Time Machine stopped backing up, stays in "preparing..." for ever

I contacted support, escalated to a second tier, he had me move the backup folder on the time machine disk to a new sub-folder, and letting it run... currently still "Preparing Backup..."

however as I was on hold and later as I was following the instructions, I looked at the log files (via the console app) here is what I found out:

there are several crash reports from cfbackd, every hour to two hours overnight while time machine was showing "preparing backup..."

in the cfbackd.log there are several errors:

E 2021-05-10T12:45:03 Will crash daemon because of memory exhaustion. Process resident size: 2694135808, virtual: 8144347136.

E 2021-05-10T12:45:15 Can't sweep spoiled records from GR at /System/Volumes/Data. Sqlite error: SQL logic error, code 1.

which is why I am first trying what Apple Support suggested... move the backup folder to a subfolder, and let it re-create a new backup from scratch... still on "Preparing backup..." on the first disk.

I was able to make ONE backup to each disk... but no more...

tmutil deletelocalsnapshots and tmutils deleteinprogress twice, I removed the disks from time machine, boot into recovery, disk first aid on all three disks (internal and both external) twice. then boot into safe mode, added the disk back to the time machine, then started the backup, which took nearly forever to complete.

however, now that I re-booted into "regular" mode, it ain't backing up no more :-(

getting "preparing backup..." again for over eight hours now.

will do, (Costco has a good return policy...)

but I really do not believe that this has anything to do with the external destination disks.

the reason I believe that it is not the destination disks, is that I have two of them already, very highly unlikely that both, at the same time went bad.

espcially since one of them is a less than year old server class disk Seagate Ironwolf Pro (ST8000NE001) with a very high MTBF

and the other is also about a year old consumer Seagate from Costco

I find it very hard to believe that both will go bad just like that, and on the same day (April 29 was the last date of automated backup)

and, I was also able to get a semi-automated backup to both disks last weekend, after I did a disk first aid, deleteinprogress, etc, see above

Thanks, that will be my next step.

but what I find interesting, is that I have two external alternating backup disks, very strange that both got "messed up", yet both passed multiple disk first aid.

and even though I don't quite understand the crash reports and the error messages in the log file it seems that there might be a problem with the time machine software itself.

go figure this one out:

uptime of over 7 days. i.e. no recent reboots. have not done anything, since I was busy and every now and then I just looked at it still "Preparing Backup..."

then today, I just happened to check again and Time Machine just started backing up, on its own, it is in the middle of backing up to the first external disk... no more "Preparing Backup"

I just hope it lasts.

no go

still "Preparing Backup..." after two days... even after I moved the backup folder into a new folder (as instructed by Apple support)