SharePoint error 0x81020030 Invalid file name

If you try to upload a file to a sharepoint document library, and get an error about invalid file name or url, it could be that your problem has nothing to do with the file name - it could be a problem with a corrupt column that has been added to the document library.

In my case, I had to remove column by column until I found that the issue goes away if I remove a calculated column that I created using a feature (using CAML), and the XML definition of the column was missing a critical attribute - the ResultType attribue. Adding that attribute to my XML solved the problem.