Cadiboo

Can you please explain this and this? Don't keep lists of your objects around.

Pls do. This may be the cause of your problem, but the actual error happens later and isn't printed to the log. If you can post your debug.log (Twitch doesn't generate this by default and you need to edit a json file to get it to) that would likely be more helpful. I don't remember how to edit the json file to enable logging, so please provide the launcher_log.txt from .minecraft instead. This is completely speculation, but I got a similar error with my mod where it tried to use code from Forge that wasn't available in the version that the user had installed. Updating your Forge to the latest version may fix this.

I would look at how the end portal is rendered

If you did this properly, then this isn't a Forge problem.

The Tardis mod does something like this. I think that you should set the renderViewEntity to an entity in your camera's position, render everything to a frame buffer, set the renderViewEntity back & then render your frame buffer to the main mc one.

It changed in the 1.13 rewrite. Use different launcher profiles with different game directories instead of versioned mods folders.

Don't do this. Mods are only loaded from the mods folder.

https://mcforge.readthedocs.io/en/latest/effects/sounds/#playing-sounds

Problem with OTG

Yes please. You can do a find + replace on your name in the log before posting it if you want. Please upload it with GitHub Gist or Pastebin.

Run the game

Please post your logs as described in my signature and the EAQ

No

You can use an AT or Reflection

How long have you left it running for? Do you have a good and stable internet connection?

If you have java installed, its as easy as double clicking it. You can download Java from https://www.java.com/inc/BrowserRedirect1.jsp?locale=en If you're on windows and another application has hijacked the .jar extension you can use JarFix (https://johann.loefflmann.net/en/software/jarfix/index.html) (Download: https://johann.loefflmann.net/downloads/jarfix.exe)

override getShape and return something non-full.

Yes You can create them with VoxelShapes.create. Yes You can do it by checking the entity in the context passed into getCollisionShape. return VoxelShapes.empty() from getCollisionShape or calling doesNotBlockMovement on the Block.Properties you pass in to your block.

Pls post your logs

Unfortunately thats exactly what it is. They're also (mis)using their own config system for it.

have a dummy recipe in your data folder under the Minecraft namespace

Thats not going to work, thats extremely old code targeted at a very old version. If you don't already know how to write a coremod, don't. You need to have extensive knowledge of ASM, IJVM code and how the JVM works to write a coremod. Tell the author to fix this issue then.

Since 1.13 the @Mod annotation only has a single parameter - your modid. Everything else goes in mods.toml.

gradlew genEclipseRuns should be run after gradlew eclipse.