I also got it running and it is actually way easier than I originally thought:
You need to download fabric from fabricmc.net/use/ (that is the direct download page) and select "server" at the top left, right next to "client", select the Minecraft- and fabric version (for the fabric version, the the most recent one is recommended) of your choice and download location on your device and press "install".
After downloading the "fabric-server-launch.jar" file, a new window will pop-up and unless you already have the "server.jar" in the exact same download-location, it will ask you to download it. Confirm this by clicking on "Download server jar" and after it's downloaded, press "done".
Now, go to your Server Webinterface and stop the server.
Scroll down until you see "Change game" at the bottom left.
Search for "Minecraft (Modpack)" and click on "Install".
Now go to your file browser, for example FileZilla, open the "minecraftotherpacks" folder and copy the fabric files into it (only the "fabric-server-launch.jar" and "server.jar" file and not the file you downloaded on fabricmc.net).
Delete the "minecraft_server 1.16.5.jar" and "minecraft_server.jar" files and rename the "fabric-server-launch.jar" to "minecraft_server.jar" and that's it.
I would, just to be safe, when starting the server for the first time, do it without any mods.
I hope that this it not too long xD
When updating a FabricMC server (Forge Vanilla with custom minecraft version) you will have to manually change the java version from v8 to v16. THis is due to a change in the 1.17 version of Minecraft. When changing the version in the General settings tab in the webinterface (and save the changes of course) the forcejava8 file in the minecraftotherpacks folder doesn't get deleted. I consider this as beeing a bug. Just wanted to point that out.