If you google this error, you will find out that there are a whole bunch of reasons why this error happens. The only way to find out why it gives that error is by going into the transport logs for your frontend. 

I guess not many people use IIS Reverse Proxy and NGINX or Apache is the more popular option. Nonetheless, a reverse proxy is a reverse proxy. 
It just requires a bit of translating to make it work.

Windows only allows you to extend an existing partition if there is free unallocated space directly next to it. This is most noticable when you try to extend the C:\ drive but the option isn't available.

When you change the GPO with your Folder Redirection settings to a different file server, no matter how many times you GPupdate it the settings will never apply.
This is because if you manual folder redirect on the local machine, it will ask you if you want to move the files over to the new location. The GPO can't do this when you change paths and will never process it.