I have a somewhat similar problem on Ubuntu Gnome 17.04. For me Enpass doesn't close but simply it has stopped accepting it. This is after my filesystem got full. Enpass simply stopped accepting my password. Still the main problem I am having is that it's impossible to remove the package. I tried ./EnpassMaintenanceTool and manually removing the Enpass folder but when I reinstall it keeps asking for a password. I really like this program and I appreciate having a Linux version. I hope this can be resolved asap since although I have backups of my keys, I am locked outside the program.