Just updated my iPhone 7 to iOS 14.
Before, the BT Cloud app was working perfectly.
Now it keeps crashing.
With iPad OS 14 there is no problem.
For the moment I have deleted the app from my iPhone, as the same pics can easily be backed up on my iPad, using the BT Cloud app
It's just annoying and needs fixing.
I'm using a Xr and just installed iOS14 and it seems to be OK, so far. You could maybe try re-installing the app.
Not much consolation, but you can undo the latest iOS update for your iPhone by connecting to a pc via iTunes. Then wait for a further release....there are some comments in the tech news, if you search.
It would be wise to put a query to Apple Forums for official advice on how to revert.
I updated mine today and changes appear to be cosmetic, with some features which hopefully can be disabled.