DaemonUmbra

What version of Minecraft are you using?

Care to provide more info on how you're trying to generate these islands?

Show the error please?

Have patience...

1.12 is no longer supported on this forum. Please update to a modern version of Minecraft to receive support.

Screenshots please?

Yes, that's relying on your PATH variable to find Java for you, and its pointing at Java 15. If you have both Java 15 and Java 8 installed you'll need to either modify PATH to point to Java 8 or you'll need to specify the full path to the Java 8 executable instead of relying on PATH.

What are you talking about with maps?

Are you tabbing out during load?

Please be more specific. Do you double click it and it doesn't do anything? Do you double click it and a console window opens for a split second? Do you run it via console and it crashes immediately?

Please provide debug.log (more info on where it is and how to provide it in my signature below, just expand the spoiler)

1.12 is no longer supported on this forum. Please update to a modern version of Minecraft to receive support. Edit: and just so you know this has no bearing on the matter

Well iirc the change between .3 and .4 was purely just Mojang adding MS auth, so you should be able to use your mod on both versions as long as you haven't prevented it from working via your mods.toml

You installed AutoRegLib for 1.16 on 1.15

You should only ever download the installer. As for it opening in 7zip, try Johann Löfflmann's JarFix utility.

Please provide a screenshot of your launcher

I don't know why, I only know that it is. You can switch to Java 8 by fixing your PATH environment variable, or by uninstall all Java versions other than 8.

As a mod dev or a mod user?

Not sure what kind of response you're expecting or why you posted in this particular subforum (thread moved to a more appropriate subforum).

Forge is not compatible with Java 15 because of a library that is no longer shipped as standard in that version

1.12 is no longer supported on this forum. Please update to a modern version of Minecraft to receive support.

Something involving the server not having control of the file that contains the world save

The name(s) of one or more of your mods indicate you did not get them from a reputable source. Please visit StopModReposts to learn why this is a bad thing.

That last line is not the part you should be worrying about. Try removing Optifine.

Hpbow, please do not hijack other users threads, especially when your issues are not even similar and you're using a version that's not supported anymore.