For any bug reports please provide the following details as well,

• MC Version: 1.16.5
• C&B Version: chiselsandbits-0.3.4-RELEASE.jar
• Do You have Optifine: Yes, however it is a server stall/crash.

https://pastebin.com/BiKQ5Q8W

Arclight core problem.

They seem to be accessing block light strength during world load.