I think the inclusion of including sliders.csv in mods has introduced a bug in 0.5. Currently, if you enable a mod which doesn’t include a sliders.csv file, the game crashes when you try to start a mission. It works fine if there is a sliders.csv, even if that’s just an empty text file.
You can test this by creating a blank test mod that contains nothing. If you do this, the game will crash when you try to start a mission. Then add in a blank sliders.csv to /data/simulation/ and the game starts fine.
To modders: For the moment, this can be worked around by just including a blank sliders.csv in your mod if you’re not intending to include any actual new sliders.