Well, it's funny that this issue should crop up, again. It happened recently in another forum, but as it seems to be restricted to iOS devices and a particular third-party theme, so what has changed? OK, I am assuming that as Android devices have not been mentioned, there is no issue there. So have you changed anything in your Moodle? Have you, or your service provider, changed anything in the server environment? Has Apple run an upgrade for iOS that has a known or undocumented issue? I don't use iPhones or iPads so wouldn't know, but you might be able to find out.