So I narrowed it down to Elemental Dimensions: https://minecraft.curseforge.com/projects/elemental-dimensions?gameCategorySlug=mc-mods&projectID=248539
Client locks up and doesn't even get to creating a crash log, so I had to copy from the client...
log: https://gist.github.com/ruyanve/c0a9cf806930e39e87918373e4a90bbc

That is at the moment the multi dimension thingy, that he is working on:
Caused by: sun.reflect.generics.reflectiveObjects.NotImplementedException

He is working on that.

Latest update v15 fixed my Harvest festival load up issue, seems to have been the same error you got there B) Test that version out 👍

Will do, I'm on the road for a couple of days so I can't really test.
Thanks mate!

Thanks for the report and the assistance guys, please check out v15 or v16 from https://minecraft.curseforge.com/projects/open-terrain-generator/files and tell me if that fixes the issue.

Closing for now, feel free to reopen if the issue is not solved with v15 or v16