Just upgraded to High Sierra and monodevelop stopped opening all together - was using 2017.3.0b5 - so installed the latest beta 0b8 . Monodevelop started opening now, but it will not let you double click a script and open the file - Monodevelop will open but not load the script - it also freezes up Unity often during this. To open scripts, I’m having to use the File-Open within Monodev
Update: it only does it if MonoDevelop is already open, but i noticed it will open the script from Unity if I’m not currently running MD.