My issue #913 is a duplicate of this issue, but with version 0.16-410-g7d8504e,
so I'll continue here.

I looked at 'lua_yieldk@lua/ldo.c' and without really understanding the
internals... my best guess is that the error is triggered when a coroutine
exists but is not currently running.

The error should come from the 'luaL_dostring' inside 'run_lua@src/lua.c',
called from 'complete_mission@src/mission.c',
called from '_complete_quest@src/lua/luaFD_tux.c',
called from 'end_quest@lua_modules/script_helpers.lua',
called from 'node30@data/storyline/act1/dialogs/Ewalds_296.lua'.

To know more, pretty_print_lua_error would need to print info about the stack
contents (of all coroutines?).
No idea how to do that... so I'll submit a save file right before the error (you
only need to speak with the droid).