SharePoint Online Troubleshooting: The URL 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

Problem

User reports that when she attempts to upload any document to her site's default Documents (.../Shared Documents/) library, she is presented with this message:

The URL 'Shared Documents/...' 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.
When she attempts to create a new folder in the Documents library, she is presented with this message:

Cannot create folder 'sites/.../Shared Documents/...'.
She was able to do it about a month ago but now cannot.
The document library is in the user's SharePoint Online site that she created herself (through her organization's O365 subscription).

Verified user's experience through direct observation.
Found user's account is a member of the site's Site Collection Administrator group.
Found Documents document library inherits its permissions from the site.
Able to repeat user's experience connecting to the site using own account.
Found default value for library's user-created column, Meeting Date, of type "Date and Time," was set to "Wed Oct 03 2018", which was not in the expected format.

Reset Meeting Date column default value to the expected format, or "10/3/2018".