SandBoxie in general is very buggy.
Windows 10 only certain versions sandbox fully runs with.
On top of that. The Gmod running in sandboxie will most likely crash more than your main game, as you are running a second instance of it, using up to double sometimes more of the normal resources of your pc.
The best options to run an alt account is to either:
1. Use second computer
2. Create Virtual Machines to run Steam Only on.
-Anon