Internal error: Database; UNIQUE constraint failed; UPDATE .....= ? WHERE

[photofan1]

Lightroom suddenly shows "No photos in selected Folder" on all 19 of the folders on an external drive that's 5-weeks old. The previous import path in navigation panel shows nothing. I think I right-clicked on one of the folders and got this error message:

"An Internal Error has occured: Database C/Users/XXXXX/Pictures/Lightroom/LightroomCatalog-2-v10.lrcat: UNIQUE constraint failed: AgLibrary.rootFlder, AgLibraryFolder.pathFromRoot

Statement: UPDATE AgLibraryFolder SET rootfolder = ?,pathFromRoot = ? WHERE

I had exported one folder to create a new catalog of just that folder. Though it was after that, that I imported the two latest folders. I don't have the ? showing the folders are missing, and I can't reimport those images, because they are already there, without deleting the folders.

Any help is greatly appreciated. 

[photofan1]

Well, I opened catalog backup from before I exported one folder as a catalog. I'm importing the two new folders that were imported after the last backup, and it looks like I'm good to go. I don't think the issue was a "Synchronize Folder" issue or the Capitlzation issue I've been reading about. Maybe the exporting of the one folder as a new catalog messed up the path. Not sure, but if any thoughts, they are still welcome. 

Thanks!

[photofan1]

Well this is frustrating. So, after last week's problem, I lost a little bit of time and work but after reloading the backup catalog, I was fine. That occured after I went and used a Mac with a small separate catalog of one folder, with a few thousand photos. 

Today, Lightroom Classic was running slow. I downloaded and installed a Windows update and updated Lightroom Classic, then had problems. In my huge, main catalog, my external drive now said "No photos in selected Folder". I did not want to try a backup catalog because that would have been giving in and losing several hours of work. I tried multiple versions of my large catalog and no luck. I went and used the external SSD on a Mac with a small catalog of one folder and a few thousand images, then came home and still had problems with my regular catalog. My previous import had 24 images from my last import, my images were still on my external ssd, they hadn't moved, but still got message. I finally deleted one folder that had about six images in it. I then re-imported the folder and voila'!!! Suddenly, Lightroom Classic had seemingly, suddenly reconnected to all of the folders and I was good to go!!!

Very frustrating. Not sure what the issues is, but I'm going to back up catalog now!