After Send Now is clicked on an Email Digest when performing any other operation, session is expired and user has to log in again. Not a show stopper but certainly a nuisance. ERPNext 7.1.8 Frappe 7.1.7
If you are logged in as Administrator in your account then set simultaneous sessions as 2 for admin.
You can achieve that by following below steps:
Setup>> User>> Security Settings- Section>> Simultaneous Sessions>> 2
Hope this helps.
Do consider upgrading your ERPNext instance as well. The ERPNext version in production is 7.2.8