Thanks for your reply Bryle – appreciated. I took over the development and support activities for the affected site last week, and in the last few days I have encountered a few more critical errors on the website (including critical errors on the theme customizer, and issues with two other plugins), so it appears that the error is far bigger than UpdraftPlus. This is leading me to the conclusion that there is a corruption of some core files on the site, and I am coming to the conclusion that the only way to resolve the issues and be confident of having a fully operational site is to rebuild the site. And to answer your question / suggestion, I have already done both of the things that you suggested, and it has made no difference to the issues. Thanks
