A friend of mine has been wanting to join me and a number of friends in playing modded minecraft servers. I have a hosting service for my server, and NO OTHER FRIENDS of mine have had this issue my friend faces, we'll refer to him as Salt.
We have tried playing TWO modpacks in particular, Vault Hunters Official, and Valhelsia 3 (The one we are currently trying to get him into.) In both instances, Salt CAN connect to the servers, he struggles to STAY connected. When Salt connects, his connection is perfectly smooth, but give it 10-45 seconds and he WILL disconnect; the typical "a connection was forcibly closed by the remote host."
The hosting service I use is Akliz.net.
After extensive troubleshooting we've eliminated possible causesof the issues; You'll have to take my word for some of the points because I don't think explaining the troubleshooting process of every point is very time-efficient.
The issue is NOT Salt's computer specs.
The issue is NOT firewall settings or the like, as Salt is able to stay connected to any Vanilla Minecraft server with perfect ping (we specifically used hypixel for this example). We have also tried many of the typical troubleshooting methods recommended online for not being able to connect to minecraft servers for good measure.
We've wondered if the chunks he is loading take too long for him to load as he's connecting, so we've asked him to reduce his render distance to 8 chunks or less. (we've tried this multiple times and it doesn't seem to have had any effect)
If you have any clues or questions, please feel free to ask me. I should also point out in the time of writing this we're seeing how reinstalling curseforge may work for him. (also deleting the modpacks before hand for safe measure)