A Microsoft file hosting and synchronization service.
Same issue, i wanted to uncheck a folder cause i didn't wanted on my computer anymore and the same error message appeared, so i went to the web in onedrive, create a new folder and put everything that was on the old folder that i wanted to uncheck from my computer onto this new folder and then deleted the old folder from my computer and check on the web that was deleted, it worked but is just a trouble to do all this!
Also If your folder is inside another one, you have to move everything from the general folder, not only the one inside it 😖, i was lucky mine was just one folder inside another that was on my computer, so i didn't have to move the other ones. I hope you can understand, cause english is not my first language, any questions feel free to ask!