Hit By Toxic Atmosphere Despite Being On Habitable World

[Chesoku]

Sounds like your airlock is open and the spacecraft is depressurized. Maybe your airlock is an automatic door and you have rats on the ship opening and closing it?

The message about toxic atmosphere is an old issue that only happens when you receive atmospheric damage while inside an atmosphere. Since your ship is depressurized for some reason, you are taking atmospheric damage while you are technically inside an atmosphere. Hence you get the confusing message.
I guess that old bug report I made was on the old forum, so Haxus should have a log of it in his issue tracking database.

My airlock was indeed a automatic door, and lead right into the cabin. So workless crew might've triggered it to open
I fixed up my ships design and will report back here if sudden deaths happen again.