You should probably report to the mod author once you've discovered which mod is breaking your server by singling it out, you can probably then continue playing by simply disabling it.Any way to fix it, then? Or should I just report it to the mod author?
-> To all the people having issues with the r15 builder, apologies for the wait ,we will try to figure the fix out tomorrow (currently it's 2:40 AM so we'd like to catch some sleep first)
(In case you want to try and hotfix this & not using game launcher , duplicate your terraria.exe and rename it to tAPI.exe , the builder is probably failing to work due to not filing tapi which it relies on)