Minecraft version 1.16.5
Modpack: Valhelsia 3 version 3.3.2
FluxNetworks version 1.16.5-6.1.7.12

Happened once after that its been a routine, i stopped the server, came back next day, server freezes at "preparing spawn area 100%" never lunches.

Took me hours to figure to problem. from corrupted chunks to deleting every file by its self. i deleted the whole data folder in world and it worked. then started deleting every single .dat file from data folder until fluxnetworksdata.dat.

Now every time i stop the server i have to delete fluxnetworksdata.dat from the world folder to be able to lunch the server again.

Hope you can help.

So i fixed this problem for anyone in the future. Found out issue causing this was chunk loading feature by flux networks.
It only happens when the server tries to load a chunk that is far from where you left the server.

I just stopped using chunk loading and got a chunk loader mod (Chunk Loaders by SuperMartijn642).