A missing web.config in those directories would give a 404 error as there would be no permissions to serve the static file - rather than a 500 error processing the web.config files in the static file directories - you will need to enable Trace Logging for
a more detailed error report.
I have encountered this problem on IIS following a change to the web config files (Orchard 1.4.1) to include a cache setting for the HTTP headers - see this discussion for a possible fix to the problem - http://orchard.codeplex.com/discussions/397724.
As I do not have permissions to alter my server settings I had to revert to the web.config files from Orchard 1.3.9 - note these are the web.config files located in each directory where static files are served from, not the root web.config file. You may
be able to change your server settings to allow this section of the web.config:
<clientCache cacheControlMode="UseMaxAge" cacheControlMaxAge="7.00:00:00" />