"Find My" unable to connect to server

For some months I have been unable to track the locations of family members using my iPhone 6s. I can successfully track them using the browser iCloud interface, and my iPad, and they can track me using their phones, so I think my account is fine, and their accounts and devices are fine. It just seems to be my phone that has the problem.

This started following an update last year - sorry I can't say which version I was updating from/to; whenever I tried to see others' locations, I got a popup saying:

Find My Unavailable

Unable to connect to server.

Please try again later.

While investigating this problem, I noticed there was a new concept called the "family group", so we created one of those, all joined a group, but nothing changed. We also re-shared locations with each other, but it didn't change anything. All the O2 shop staff could suggest was deleting the app and reinstalling. As far as I can see that's not possible with a built-in app, so that didn't help. I wondered if it could be a mobile data restriction, but this happens even when I'm connected to Wi-Fi, so there should be absolutely no mobile data limitations causing it.

I just left the issue for a while, as I had other priorities on my mind, but today I checked again. After updating to 14.8, and restarting the phone, the problem still persists. I updated to 15.0.1, launched "Find My" and got exactly the same popup as before saying:

Find My Unavailable

Unable to connect to server.

Please try again later.

Any ideas how to resolve this?