invalid END header

[JHubi1]

Hi there you all,
I was trying to use SKCraftLauncher for my modding projects, because it seems to be kinda robust.
I followed the tutorial, tested and debugged (everything works fine). All files seem to be just fine, but the JAR files are corrupted. I'm not able to find somebody with the same issue, so I'm just asking about it here.

I use fabric as my modloader on v1.15.7 with 1.20.4, if that's important. The packages file is stored as PHP.

This fabric error is thrown, when I try to open the pack by clicking "Launch":

(The whole page is full of the same error, for every mod I have in my pack)

The console window provided by the launcher outputs this: https://pastebin.com/aY3R5RCE

The jar files seem to be corrupted, I accidentally clicked on one (windows) and it showed Invalid or corrupt jarfile. It's not my server's vault, though. He has been serving files for me for a longer time. Additionally, when I right-click on one of the files and open it in text editor, I can see that it is not an error message the server gave back.

I believe, that SKCraftLauncher is not putting the data rightly back together. The strange thing though is, that the Fabric executable jar seems to work fine, and that consistently. I downloaded the pack around five times. And every time it worked. That's strange. Could somebody look into it? Thank you already.

~ Jakob