I did a search for my issue in the phpBB forum. I found lots of threads on the topic and tried a few of the suggestions. They did not work.
I opened my own thread here https://www.phpbb.com/community/viewtopic.php?f=661&t=2594811&p=15724681#p15724681
Still no solution. One of the support team suggested I open a bug report in Tracker. Particularly as many people are having the same issue.
So this is why I am posting here. We would like to use phpBB but if we cannot use it onc installed it's very frustrating.