Just wanted to bump this thread because I am having the same issue and anything I try wont fix it. I've made sure my entire pc has everything up-to-date, I've completely uninstalled Curseforge and Overwolf about three times now. I have also tried switching my connection and using the neighbors Wi-Fi to see maybe it was a restriction due to our network preferences. No antivirus software installed, I even tried disabling my firewalls and trying to install the modpacks. I've figured its only certain modpacks that are related to this issues such as "All The Mods 6" though I can download "SkyFactory 4" no problem. Though skyfactory is Minecraft version 1.12.2 while All The Mods 6 is Minecraft version 1.16.5. Which maybe the Minecraft version somehow correlates to our "The underlying connection was closed: An unexpected error occurred on a send" issue. I cannot create my own modpack profiles on Curseforge with the Minecraft version of 1.16.5 though I can with the 1.12.2 version. Hopefully this helps determines what is going on with this error because of as now I cannot play the modpacks I want to with my friends or heck any modpacks at least in the Minecraft version 1.16.5. Anything would be helpful!!!