Hi Ken,
Thanks for the update. We upgraded to PHP 7.1 after the problem started happening, with the hope of fixing it. The problem occurred when we were running 5.6 as well.
It looks like most of the of the Office 365 integration plugins should support Moodle 3.3, but a few of them don't currently, so I'll try to reach out to Mike to see if he has any insight on this issue.
Thanks