Forge 2780
Minecraft 1.12.2
Rec Complex 1.4.7

Crash Log: https://hastebin.com/oceboqayak.rb

This keeps happening. Turned down the structure chance to 0.4, then 0.2, but this just keeps happening Server isn't weak, neither is the client, so this seems to be the mod using too many resources.

It seems that whenever a big structure like the maze is spawned, the server freezes for a long time or can even crash. We managed to solve the issue by modifying the following values. We still get a NBT crash from some of the generated structures but that happens like every 3 hours instead of a crash every 10-30 minutes.

      # Enable hacks for a few decoration types. Disabling this may fix problems but will deactivate ReC overriding those types of structures. [default: true]
     B:decorationHacks=false
 
     # Maze generation timeout, in milliseconds. After the time is over, the maze generation will just give up. [range: -1 ~ 600000, default: 20000]
     I:mazeTimeout=80
 
     # Maximum number of reverses per room the maze generator can do. A higher number results in a better generation success rate, but may freeze the server temporarily. [range: -1.0 ~ 100.0, default: 3.0]
     S:mazePlacementReversesPerRoom=1.0

I'm not sure about the generation hacks but turning down maze timeout might solve crashes for some things. It might also help to turn down tree replacements, since some of those are rather large and can thus cause chained chunk gen as well.