It seems like files are getting double sized for some reason. In my case - jopt-simple-4.5 ..\Twitch\Minecraft\Install\libraries\net\sf\jopt-simple\jopt-simple\4.5\jopt-simple-4.5.jar after trying to launch modpack - 120KB and the file downloaded from https://libraries.minecraft.net/net/sf/jopt-simple/jopt-simple/4.5/jopt-simple-4.5.jar directly is 60KB. Yesterday got near the same issue with another modpack, have to download jna-4.4.0 file manually from libraries host to run the pack.
The workaround: Download file using link from error description and put it manually to the folder from the error description overwriting existing one.
Sergei Ammosov
It seems like files are getting double sized for some reason. In my case - jopt-simple-4.5 ..\Twitch\Minecraft\Install\libraries\net\sf\jopt-simple\jopt-simple\4.5\jopt-simple-4.5.jar after trying to launch modpack - 120KB and the file downloaded from https://libraries.minecraft.net/net/sf/jopt-simple/jopt-simple/4.5/jopt-simple-4.5.jar directly is 60KB. Yesterday got near the same issue with another modpack, have to download jna-4.4.0 file manually from libraries host to run the pack.
The workaround: Download file using link from error description and put it manually to the folder from the error description overwriting existing one.