No crash log, just strange messages before the server shuts down unexpectedly. Not sure where to go from here, especially since it's a warning message...
Versions:
Minecraft: 1.16.5
Tinker's Construct: 3.0.4.197
Environmental Creepers: 1.6.0

[00:48:31] [Server thread/WARN] [environmentalcreepers/]: Invalid entity class name (class not found): 'appeng.entity.EntitySingularity'
[00:48:31] [Server thread/WARN] [environmentalcreepers/]: Invalid explosion class name (class not found): 'slimeknights.tconstruct.gadgets.entity.ExplosionEFLN'

Those are just warnings due to the old default backlist values. I should probably actually remove or update those default values, as they changed since 1.12.2 anyway.

I'm able to load into a single player world, and also start a local server and log in with a client with EnvC and TiC installed (no other mods, except for Mantle). So there must be something else involved as well.