Deleting the config solved the issue. Unfortunately, in my haste, I did not save the config. Crash report attached.
crash-2020-08-12_17.17.13-client.txt

I'm aware of the issue, the 4.0.0alpha to 4.0.0beta config file converter currently crashes after an attempt to fix it for Java 8 and it will either be fixed or removed for the next release, the only solution for now is not having an okzoomer-next.json5 file in the config folder

How about just renaming it like suffix "-old" instead of converting?

How about just renaming it like suffix "-old" instead of converting?

Not a bad idea, I'll consider that

Fixed on 4.0.0-beta.3, the conversion now works properly, but only on Java 11+ due to issues, however, it won't crash anymore on any Java version (including 8-10). On an unrelated note, the conversion leaves a backup for the old config file.