Ohh thanks for this. So if you never hit endgame to unlock the quests you dont need this mod. Thank goodness. Jokes on me using an age old modlist

Yep! That's what the 'unknown mission' error is caused by-- uninstalling the mod when you have some of its missions unlocked

I uninstalled RtE without this mod, and my game seems to be fine. It must become a problem only after you reach the end-game that triggers RtE missions to become available.

I have no idea to be honest, I've never used Return to Earth. Try uninstalling this mod and checking your available missions in SAIL! The crash happens because RtE adds SAIL missions, and once the mod is removed, the game dies because it can't find those missions when you talk to SAIL. This mod adds dummy missions with the same ID as those missions, so the game doesn't die.

Is it possible to unlock any of those missions before completing the main quest line? Because I haven't, so maybe it's safe to just uninstall Return to Earth without even using this mod.

@kenanthebarbarian you have to keep it unless you no longer have any characters which have the Return to Earth missions unlocked in SAIL

Can I uninstall this mod after removing Return to Earth, or must I keep it?

You can remove it with character editing

i wish this would also remove the mission, but sails hardcoded so im not sure if thats possible