I had to disable two extensions today to allow the site to work correctly. Today for unknown reasons the forum crashed and generated internal server error 500 and eventually after some stressful hours and trying a multitude of possible fixes eventually I manually deleted the contents of a cache which restored the site's functionality. I also disabled the two extensions: board announcements
and collapse/expand the forum sections
for now because they suddenly did not operate correctly and seemed to have polluted this particular important cache.
Forgot about the first rule with these complex web site code.......clear the caches first and usually this is the fix when faced with the a blank white browser screen.
Tried to consult the pros at the web host but they are server admins and don't have a clue about phpBB so more of a waste of time but did lead my thought process to the "delete the TWIG cache and test again" idea.
I took the time to upgrade the forum's software version to the very latest just released today.