TomBenj wrote: Actually, I was wrong on the PHP version, it's 7.4,
Looks like you use cPanel with your hosting. You should be able to change your PHP version as well, right?
I recommend to change to PHP 7.3 for a while.
If you still cannot reproduce the issue on your own (since you don't own a iOS/MacOS device with safari), the error reporting is from your survey visitors. Are they all from different areas? Or is it a survey with a certain group of visitors (e.g. one company)? Since we don't know which Safariversion was used, it might be a certain safari issue which is matched by a LimeSurvey bug.
The error message "Please use the LimeSurvey navigation buttons or index. It appears you attempted to use the browser back button to re-submit a page" can be triggered by many different things. That error message is an evergreen. If the issue is always at the same page, it can be an content issue of the page. You should copy the survey and reduce the survey to just the problematic question and check again.
I recommend to switch to LS3 (even though you got these reports there as well) and to PHP 7.3. If you still got these reports, you might ask the reporter about the exact version/device.