If you get this error as while checking in a document or performing any other action in your environment, it is likely you have a database space issue:
“Cannot complete this action. The URL ‘Pages/default.aspx’ 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.”
In my case the transaction logs disk drive was full. In other cases, it could mean your database is set to read only.
You probably would have caught this if you looked at your SharePoint server’s event viewer logs.