When I did a search for the error I found a website where the following file was listed : /wp-content/plugins/wpdm-premium-packages/languages/wpdm-premium-packages.pot where the error is defined as follows :
#: wpdm-premium-packages.php:471
msgid "Calling 911! You better run now!!"
msgstr ""
Is perhaps an old file left over? I can’t currently check if that is on my web server or not and can’t download WPDM from where I am currently to check
The issue itself now seems to be resolved – the problem was, I think, that my test page was a draft and I was previewing it when that error occurred, so it wasn’t a published page. So when I saved the page and then privately published it, everything worked OK. It was just that error message seemed a bit bizarre.