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Each connection you make to BT Wifi, results in a stored wireless profile. These stored profiles can cause issues, they do on Android phones, and its probably the same for your iPhone.
There is probably a stored wireless networks screen somewhere. If you delete any "remembered" wireless profiles for BT Wifi, BT Fon, and BTOpenzone, you may find that it helps.
All you will be left with would be the current hotspot you are connecting to.
The problem seems to be that you end up will lots of stored profiles, all with the same SSID, even though the BSSIDs are different.
This would be why your iPhone used to work, when it only had a couple of profiles stored. As this has increased, you have started getting problems, as the phone cannot determine which profile to use.
Delete these old profiles, and see if it helps.
I am not familiar with the iPhone as I have Android stuff, and I have noticed the issue on my phone.
As its been suggested, can you login without the app, just using the web browser?