In case you have a file in the virtual directory with filename containing non-English characters like Umlauts e.g. ö. When we try to access the path with it is becomes inaccessible, but the files with only English characters are accessible.

You can try the following IIS settings, first one is

Request Filtering:

1. Open the iis, double click the ‘Request Filtering’ icon
2. In the ‘File Name Extension’ right click->Edit Feature Settings…’ the file ‘web.config’
3. Check the option ‘Allow double escaping’ (this option is unchecked by default)
4. Repeat all above 3 steps for the ‘default website’ (or whatever you have given the name to your site)
5. Re-start IIS.

UrlScan under ISAPI:

One of the possible causes could be you’re using UrlScan extension for IIS which is visible under ISAPI filters. It is applied to all sites by default. In our case, removing UrlScan for the site facing issue resolved the issue.