Ruins is causing a cascading worldgen error, this may be only when it works with OTG or just in general. It's not causing a crash so there's nothing to log but the server does become unplayable due to it. This is the error from the server log:

[02:26:59] [Server thread/WARN]: Ruins Mod loaded a new chunk [306, 308] in dimension 2 (Southwest Terrain) while populating chunk [306, 309], causing cascading worldgen lag.
[02:26:59] [Server thread/WARN]: Please report this to the mod's issue tracker. This log can be disabled in the Forge config.

It's not an error but finger-pointing by forge. They were sick of people complaining to them about slow worldgen when it was actually mods causing this. Ruins may "leak" into adjacent chunks when building templates. This is to be expected.