Importing an Excel spreadsheet to a sharepoint list results in Method "Post" of object "IOWSPostData" failed

A client was trying to import an Excel spreadsheet into a SharePoint list on a collaboration site and was receiving the following error:

Method "Post" of object "IOWSPostData" failed

Basically, the error is a result of Excel failing to recognize the current version of SharePoint and attempting to upload the data via a raw post instead of using the SOAP format that SharePoint 07 expects.

This error only appeared when importing to site collections within a specific web application. When the client attempted to import to their MySite, the error did not occur. Similarly, when the client attempted to import to a site collection on a different farm, the error did not occur.

All of the existing "fixes" posted on the internet for this problem involve updating a client-side file in order to force excel into utilizing SOAP for the import. An example:

http://blog.sharepointhosting.com/Lists/Posts/Post.aspx?List=399ed944%2D72d2%2D4932%2Dbc13%2Dde0586abf2d1&ID=64

Since this error was dependent on the web application we were importing to, it was clear that the solution is NOT to change the client, but to find out what was different about the web application that would result in this error.

The solution was actually quite simple, but not obvious: Create a site collection at the root of the web application (http:///), and this error will go away.

I'm not really sure why this fixes the problem, though I suspect that Excel is checking the root site in order to determine SharePoint's version. Hopefully MS will fix this in a future version or patch, as I don't think you should really be required to have a site collection at the root of any web application.