Well, for starters any folks wanting to help would need to see the crash logs. It would be helpful to post these to codeshare.io or some place like that to see what went wrong. Additionally, how many mods are in the pack, and how much ram is allocated to server and client. I personally (from experience) would recommend you not use more than 8-10 for the client.

The mod developer of that mod recommends GDLauncher as twitch launcher has issues with the mod. Apart from that, I didn't know minecraft.net hosted servers (especially Java ones)

first, don't use a readme.md file for your crash report, instead using a pasting service next time.

This is how you use a pasting service. ``` 1. open the file (crash report, log, any text file actually) and 2. hit the edit menu and 3. then hit select all, 4. then edit again and 5. then copy, 6. then goto https://hastebin.com and 7. in the large box, click in it and 8. then on your keyboard hit CTRL then V on Apple that would be Option + V, 9. then hit the submit button, 10. the page will change and but will look the same, 11. there will be a new url/address at the top. 12. copy that url, and 13. then paste in the issues here.

This seems like a lot but it will become second nature really fast and it saves those the time of trying to help you from actually having to download the file then open it as it'll just open it right in their browsers. ```

A pasting service will help preserve the format of your crash report.

Further we could get more details, and it would be easier to read.

9 out 10 times

the first few lines says what your problem is.

in this case

cpw.mods.fml.common.LoaderException: java.lang.NoClassDefFoundError: net/minecraft/client/gui/GuiScreen at

which in your case

is being caused by custom main menu which is a client only mod

in the case of

errors after the first one, it's common programming practice to not go after them, because the first one errored thus cascading failure till at some point the master program said 'f' it. simply fix the first error you can possibly fix and reload.

Removed: Better Diving, Embers Rekindled, TexFix, Craftweaker2, Minetweaker Recipe Maker, The Betweenlands, OreLib, Dynamic surroundings, Bountiful.

Crash: https://www.docdroid.net/8pM7NCm/latest-txt

Same error but with Tough as Nails

Could you try a free VPN service? Maybe give ProtonVPN a try to see if that makes a difference. The only other thing to do is try a speedtest with the server, but I don't reckon that's possible considering what type of server it is.