@ThomasWinzer . Have you seen the editional notes for the grav 1.7 changes , not sure if your current grav has been upgraded from 1.6 hence whilst the existing configuration is still working.
First the obivious things
Taken from Requirements | Grav Documentation
To get Grav running on an IIS server, you need to install URL Rewrite. This can be accomplished using Microsoft Web Platform Installer from within IIS. You can also install URL Rewrite by going to iis.net.
Have you just uploaded the mac / devolpement “grav” main directory straight up on the windows server or you have done a fresh install of grav on the windows server and then just copying the user directory across.
There has been some changes in grav 17, did you initally set up with an earlier version of grav and got it all working and did an internal upgrade to get it to version 17 ?
Environment Configuration | Grav Documentation (getgrav.org)
Are you using virtual directories and subdomains or using a domain and directory approach as with 1.7 the setup files are different now .
It is a bit unclear from the 1.6 to 1.7 the best way to setup up multi sites.
My first thought is “does not exist” maybe a permission error !
Happy to give you hand to sort it also would be a good introduction to multi site for me as well … I have a windows test wamp stacking running to I can try to emulate the issue first and if need be, possibly remote teamviewer session or I can sort out a virtual machine with windows server set up on it
Lots to go at ! lol