I've just had a student bring this to my attention.
After an iOS update, the moodle url was cleared from her device. When she re-enters it, there is an error. I've included a screen shot of the error screen.
The site is configured correctly according to these instructions: https://docs.moodle.org/33/en/Moodle_Mobile_guide_for_admins
When I check on my iPad, which already has the url saved from setting it up previously, everything works as expected.
Our Moodle version is 3.3, and completely up to date.