File History Does Not Work

There is indeed a simple fix: Replace c:\windows32\fhcfg.dll with the version before KB4601319. If you don't have it, you can download it from https://bhpark.dk/fhcfg.dll.
I use newest version of fhcfg.dkk in normal day to day work. When I need to backup, I replace it with the older version, takes the backup and restores the newest version again. And some day - by a pure miracle - I don't need the fix any more :-)

Okay.  How do you "simply" change this DLL whenever you want.  I had a quick search around to find a simple way and can't see it.  Thanks for the help.

I have the exact same problem. I am 99 percent sure it is due to the newly installed KB4601319. When I roll back KB4601319 the problem disappears and when I apply it again the problem reappears.

I am experiencing exactly the same problem since February 10, 2021. I tried a complete reset of File History: deleted configuration files, the backup folder itself and changed the destination to another external drive, and the problem still persists. Rolling back KB4601319 corrected the issue.

I had thought the problem would be resolved with a future update.

KB5000802 has been installed on my PC now but File History still shows size of backup as 0 bytes.

Yeah, I definitely did not get that. I could only verify that the backup wasn't working by going into the "more options" under Backup in the control panel, and seeing that the last backup date was Feb 12, 2021 (the day before the KB update was installed). I know the notifications work, because while I was trying to debug this whole mess I removed the File History backup by "removing this drive" for the destination drive. Once that was off, I regularly got the notification you describe.

Interestingly, once it was off, I couldn't re-create it until the KB was uninstalled ... File History was unable to connect to my network drive.

Interesting you got a notification ... I didn't. I only discovered backup wasn't working when I happened to browse my backup drive and noticed that nothing had changed since Feb. 13th, the day I installed the last cumulative update. And try as I might, I could not fire off a manual backup until the KB4601319 update was uninstalled.

The notification was that routine back-up reminder from the PC that occasionally appears in the "notification centre"

So what we are dealing with here is an update that broke quite a few things, including the backup system so you can't roll back to fix it. Hmm. One day, Microsoft will make their OS "unplayable" and that will finally be the end of it.