Also having this issue, tried with EZ Pixelmon, launcher just restarts when i get to a certain point of the launch (Certain point is the installing Minecraft assets part). I have re-installed the modpack twice. Decided to install another modpack on the off chance that it was just the 1 Modpack, installed Blightfall and same issue. Modpacks with the Twitch Launcher is working fine, as is Vanilla Minecraft.