This post is for others suffering recent issues with the back button not working
and in the hope that either Opera or the extension creator will correct the issue,
or most of all that Opera will enable a Turn History Off in the browser settings menu.
Since updating to opera version 55.0.2994.44 I have had issue with a conflict between
the extension History On/Off and the browser back button.
( https://addons.opera.com/en/extensions/details/history-onoff/ )
Currently having history on/off active means that the back button no longer works.
Prior to this update the back button still worked when the History On/Off extension was active.
History On/Off last updated July 2017 so no recent change there, but I have also reported the
issue via the History On/Off extension page