Similar error(s) can relate to a buggy plug-in. One that keeps coming up is the LogiOtions plug-in for Logitech craft keyboards.
Other plug-ins might cause similar issues.
Now the LogiOptions ones, seems to come up when you remove images, after all images get removed, up comes a similar eror message, in that case, no big deal.
Yours appears nastier.
First thing first, go in and manage your plug-ins, bring that dialog up, check for any errors.
P.S. perhaps a screenshot of your folders panel will help???
Also, outside of LR, can you view the folder in question? Finder in MAC, File Explorer in WIN.
Yes the folders are in my Pictures folder.
I did turn off 3 plug-ins: Flickr, Facebook and Nikon.
This morning, I was able to link the folders with a question mark to the actual files on my HD in the Pictures folder, so the issue semmes resolved.
So, this morning it works. Excellent.
I take it you turn your computer off at night, so now after a cold boot things work? An if you tried that before and it did not work then the following does not apply:
Shame on me for not suggesting a reboot, a cold reboot. Cold reboots can clear all sorts of odd issues up (typically because something in RAM is fouled up)