Certain PHP applications return the error “directory name is invalid” when installed on a windows server. This seems to be an issue with IIS and certain PHP versions.

A workaround is to open the IIS Admin panel, go to the MIME configuration for .php files and make sure that “Check that file exists” is enabled. This seems to resolve the problem.

Additionally, check that IUSR account has read permissions on the directory (check both IIS and NTFS settings).