Time to fix: 5 minutes
Ran into this one recently. Was trying to upload files to a document library on a SharePoint Online site. Tried PDFs, Word docs, txt files, etc. Whenever I would try to upload to the library (by drag-and-drop or otherwise) I got the following error message:
The URL [FileURLHere] is invalid. It may refer to a nonexistent file or folder or refer to a valid file or folder that is not in the current Web.
Did some searching and found a Microsoft post saying it was related to default values on date columns. Sure enough, checked the date columns on the library and found one had a default value set to “Tue Dec 04 2018 18:00:00 GMT-0600 (Central Standard Time),” which was invalid.
I removed that value, clicked the radio button for none, saved the change and voila, everything worked again.
Not sure how that invalid default value ever got set in the library, but it did.