Network Selection disabled by WiFi Manager & Wi-Fi keeps disconnecting Android 7.1.2 drops WiFi connection. 
Wi-Fi verbose log says: 

(network_selection_permamently_disabled)
network_selection_disabled_by_wifi_manager
I found out, the following WiFi Manager App was the reason: 


  Wi-Fi Privacy Police (Prevent leaking sensitive data on WiFi)


The problem disappears whenever I disable this app. Here is how to disable it:


  (System) Settings / Memory / Apps started on boot / Wi-Fi Privacy Police / Start at boot: DISABLED


So you disabled the App's bootup autostart. Now reboot Android. 
After you have Android rebooted, check again Android's Wi-Fi settings: The error message NETWORK SELECTION DISABLED BY WIFI MANAGER should have disappeared.

Wi-Fi Privacy Police was actually a useful app for long time, till I noticed the unfortunate repeated interrupted WiFi connection. Below you have a screenshot of that app's functionality: