Did you try deleting the settings.txt or moving it to a different location(while the launcher is not running)? Maybe your change broke the file in some way. You can also try to rename the whole "Documents/Paradox Interactive/Europa Universalis IV" folder to get a clean slate(also do this while the launcher is not running). If it helps, you can manually move your save games, screenshots and local mods from the renamed folder to the new folder to get them back