After updating the server to 1.4.3 (was 1.3.0) it is now impossible to connect to the server, player gets kicked.
Server doesn't crash however this error appears in the log:

io.netty.handler.codec.EncoderException: String too big (was 34278 characters, max 32767)
...
at uk.me.desert_island.rer.RoughlyEnoughResources.writeJsonPrimitive(RoughlyEnoughResources.java:140) ~[rer-2.6.0.jar:?]

Full traceback from the log: https://gist.github.com/Lgmrszd/94caf5330a838c40f367b4581c441ed4

Seems Roughly Enough Resources is the issue:
theorbtwo/RoughlyEnoughResources#60

yep, and someone made a fix back in Aug and apparently it hasnt been included in the mod theorbtwo/RoughlyEnoughResources#69

While it gets resolved by the mod dev, you can try a prebuilt with the fix https://github.com/Linguardium/RoughlyEnoughResources/releases/tag/1.19-primitiveStringFix

Temporarily solution for that is to remove the mod on a server, clients don't need to remove it in order to join.

Is it fixed in dev or in 1.5.3? I still experience this issue on 1.5.3.