I had the same issue, but was able to download the mod successfully after forcing my internet to use IPv4 instead of IPv6, Try going to your adapter settings and remove the checkbox infront of ipv6. It worked for me, hope it'll work for you :)
I don't know, but it could be that the launcher isn't setup to handle that protocol. For me the error always happened at the end of the downlaod so it may be an issue with the verification process like, a checksum that might not function over version 6 of the internet protocol. just theories tho :)
It's most likely a communication issue, so try disableing your firewall, any antivirus you have.. if you're on wireless you can try using a wired cable or the other way around if you can. You could also try using a VPN, or disableing it if you have on active.. not sure if any of these things will work, they were next on my To-Try list of things. either way, good luck and I hope you find a solution :)
So, I ended up just closing it down and going to sleep and when I got up I was able to use it. Nothing changed for me so I am thinking it might have been an issue based on their side that might be fixed now. So if you haven't checked, I might suggest going in and just clicking install now. (For FTB infinity evolved anyway)
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!!!
Guys curseforge and overwolf are in beta. This means the installation will be broken for a lot of people. So either you have to either wait for it to update or find another place for mods like planet minecraft