This is a client issue - your mate is not using the same mods or modpack
The issue refers to the mod RecurrentComplex - from the given log, your server is not using this mod - so maybe he is using a different set of mods
[minecraft/DedicatedServer]: Done (14,590s)! For help, type "help" or "?"
The server starts without any issues - what exactly is your issue?
-Xmx20G
20 GB RAM for 9 mods? Change it to -Xmx8G
Can't keep up! Did the system time change, or is the server overloaded? Running 2790ms behind, skipping 55 tick(s)
Is your game lagging? This is a common message and appears, when you walk/fly through your map and generate new chunks