If a world created in v6 had a biome ID conflict, OTG just silently allowed it to happen whereas if you upgrade that world to v8 it will try to use the same IDs and crash with the following error:
https://pastebin.com/nKJTn5Ej

Thank for reporting, do you have an example world or an easy way to create one?

There is an easy reproduction if you have the Twitch launcher. Download the Roguelike Dungeons and Adventures modpack, create a world, then update OTG and you should see the error.

Would be good to at least log a proper error, if fixing it properly takes too much time.

Logging a proper error, unfortunately I don't think I can remap biome id's for existing worlds so won't fix.

*Hm ok nm, if I understand correctly, I wouldn't have to remap biome id's for the world, but would just have to make v8 imitate the way v6 handled biome id's for conflicted biomes. If v6 detected a conflict, did it just ignore the biome completely, or did it get a different id than defined in the worldconfig?

As this is a bit of a pita to confirm (have to dig into v6 code) and may not affect many people, will put this into support milestone for now. If someone is actually stuck with this problem and can reproduce it, I'll take another look. I'll need some logs that show otg v6 registering the conflicted biomes at world load (raise log level in OTG.ini if necessary), so I can see what id's the biomes are being mapped to.

Took a quick look with the Twitch launcher and Roguelike Dungeons and Adventures modpack, unfortunately it doesn't log any info when registering biomes on world load. I think that version has the bug where the log messages only appear in the debug.log, which the twitch launcher doesn't create.

debug otg v6.log

This is a trace-level debug.log from running OTG v6, Dregora v1.5.6, and Twillight Forest 3.10.1013. Heavily edited to be more easily readable. I'm not sure if I can get more detailed logs than this.

debug OTG v6 BB.log

Here's a similar log running Biome Bundle instead of Dregora. Slimmed down again, even more than the first one.

Got a new one because I noticed the Dregora world didn't crash when loading it in v8, whereas the BB one did.

Very nice, thanks! Now all I need is a log of the crash in v8, to see which biomes its complaining about. Can also upload the world save, ofc then I'd need all the mods too.

crash-2020-01-25_16.29.46-server.txt

Here's the crash, it crashes when colliding with the first one so it's just the name of one of the biomes. Any other files you'd like?

should be fixed for v9.0_r3, to be released on the otg discord asap, thanks for reporting!