Sadly you are not the lfrist nor likley the last, looking to buy a one-way ticket out of LR City. This once bright light, looks to be burning out and as an example of Darwinian Evolutionary Therory where genetic propogation errors accumulate to the point where survial of the fittest rules come into play. For Myself, I moved back to my perpetual, non CC version of LR 6.5.1 which is stable. I will wait for an expected/promissed full release of a LR 7. Hopefully it is fuly tested and not stuff full of genetic mutation and newly broken tools... If so my ticket is purchased and it’s all aboard the 4:10 Exist Express.
When you say batch updating metadata, are you talking about writing metadata back to the files or just updating it in the catalog?
In addition to Victoria's question...
1 - Are your images and catalog on the same hard drive or different hard drives?
2 - Are any of the hard drives external - connected via USB, network shares, etc.?
3- You mentioned "The affected Lr catalog then shows errors..." Do you have more than one catalog? If so, is the behavior the same when you use a different catalog?
If you're convinced it's the Lr update, you could try to roll back to the previous version. Victoria has instructions on that process on her site - https://www.lightroomqueen.com/roll-back-update-previous/
I'd already tried the things mentioned above before writing, of course. This isn't my first rodeo as a techie for longer than Windows has been around.
Working on Windows 10 on an installed hdd, I had been adding captions to 40 or 50 photos at a time in LR, which had been lost when I renamed the folder containing the photos because LR doesn't add metadata to the files by default. Then, in the middle of the process, LR automatically installed the 2015.8 update and shut down.
This left me with a folder of damaged photo files. LR then declined to open the catalog (actually, declined to open itself after choking trying to read the files) but instead of closing it froze all PC functions. First time I'd experienced that kind of a freeze since Windows XP.
I've scanned the hdd for errors and repaired it. I've reinstalled LR, new and previous release, but by default it wants to open on the damaged catalog. I don't want to get rid of the catalog, as it contains adjustments to much of my work, especially in the past month before I got a chance to back it up. At this point, though, I don't see that I have a choice.
My disappointment with LR mainly centers on bad default choices that need to be made manually every time LR updates. It seems pointless to enter metadata if the information doesn't stay with the file, but that's one bad thing LR does by default. Another, and my main problem, is that LR updates automatically by default, even in the middle of using it. I'm sure I could find other things that tick me off if I gave it more thought.
1 - I could be wrong, but I believe you have to initiate the Lr updates. You get notified, but I don't think/have never seen one install automatically.
2 - You can tell Lr to add the metadata to the files automatically (not just store it in the catalog until you initiate the "Save Metadata to File" command) by going to the menu and clicking Edit --> Catalog Settings --> Metadata tab --> check the box next to "Automatically write metadata changes into XMP". It can have an affect on performance, but also could save your bacon in a situation like this, where your catalog backup isn't current.
3 - If you rename a folder outside of Lr (not recommended), it should show up in Lr with a question mark next to it because Lr can no longer find it. If you right-click on that question marked folder and choose "Find missing folder", you can navigate to the folder you renamed and Lr will reestablish the links and you won't lose any work. Everything you've done in Lr, like add captions, will still be there, even if you haven't written the metadata to the files themselves yet.
4 - If your catalog is truly corrupt, I don't have any suggestion other than to restore from a backup. Regretfully, if you haven't backed up in awhile, you're stuck with whatever you've got. Nobody likes the bother of having to backup, but if you're not doing it regularly, you're the one taking the risk. That's one of those things that's on you, not Lr.
5 - I'm not sure what "bad default choices" you're referring to when Lr updates. I've been through about every update out there and don't know that I've ever had it change anything that I've then had to go back and change manually. If your preferences are being overwritten/changed with every update, there may be something else going on.
I understand that none of this helps your current situation, but maybe something will help down the road.