I have the Dashboard and Smush plugins installed (and have been using them, with no problems), on a client's website, hosted on WPEngine.
Recently, they switched to php7. Since then, I noticed the WPMUdev Dashboard plugin has logged me out, and the Smush plugin is no longer Pro. I tried deactivating the dashboard plugin, so I could delete and reinstall it. I got the wp-admin notification that the plugin was deactivated, but it would not show as deactivated in the active plugins page. I tried selecting it, and using the Delete action, but it did not work. I FTP'd in, and deleted the plugin. Then, I reinstalled, reactivated, and logged in. Everything seemed fine, and Smush was again showing Pro.
I began the task of smushing the images. After a day and a half, the task had completed. Then, I noticed Super Smush had not been selected. I went to enable the option. When the screen refreshed, I again no longer had the Pro version. I went to the WPMUdev Dashboard and, once again, it had me logged out, and I was unable to log in.
This seems to be some problem, related to php7 and/or WPEngine, with your plugin. As stated earlier, before the move to php7, I had no problems.