If you do a bq_admin complete # in the server console without a name at the name, it will attempt to complete the quest for NULL, which will crash the server on its next autosave (that ~30 second auto-save interval).
Doing it from ingame, will just complete the quest for yourself, and I seem to recall from earlier versions that if you used the command without a name in console, it would complete for everybody.

Discovered when I tried to set up Discord Integration commands to complete quests, because we're bad at making quests that work ¯\_(ツ)_/¯

Tested on Better Questing 3.5.296 + Standard Expansion 3.4.154 in an earlier version of Enigmatica 2: Expert Skyblock. First discovered on Enigmatica 2 Expert 1.59a (using BQ 3.5.295)

Crash log: https://pastebin.com/6GYqmQ6Z
Latest log: https://pastebin.com/tEDvumcK - From when the server was fully started

( also, isn't "Back ups" one word? ;) )