Just tried to setup email notifications in plugin + all appeared well, when I saved settings.

Then did a plugin update on site + no email flows.

I’m using the standard MailGun plugin, which plumbs itself into PHP, so all other site mail is working, just not Activity Log.

Also, I notice a good bit of debug log spew which seems to be related to this plugin, which you can likely see by enabling WP_DEBUG + xdebug.scream (for deep detail).

My runtime environment.

# lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 17.04
Release:	17.04
Codename:	zesty

# php --version
PHP 7.1.12-2+ubuntu17.04.1+deb.sury.org+2 (cli) (built: Dec  7 2017 20:12:34) ( NTS )
Copyright (c) 1997-2017 The PHP Group
Zend Engine v3.1.0, Copyright (c) 1998-2017 Zend Technologies
    with Zend OPcache v7.1.12-2+ubuntu17.04.1+deb.sury.org+2, Copyright (c) 1999-2017, by Zend Technologies
    with Xdebug v2.5.5, Copyright (c) 2002-2017, by Derick Rethans

# wp core version
4.9.1

# wp plugin list --name=aryo-activity-log
+-------------------+--------+--------+---------+
| name              | status | update | version |
+-------------------+--------+--------+---------+
| aryo-activity-log | active | none   | 2.4.0   |
+-------------------+--------+--------+---------+