Ate you using giffgaff.s data to open the app?
I did recall I had a brief occasion when it wouldn’t open
I connected to a wifi source briefly
It opened ok
Then when I went back to giffgaff,s data the app opened as usual.
What phone software are you using?
Is it up to date?
Try a reset of network settings
(be aware this will erase your saved wifi passwords, it won’t erase or affect anything else)
And click reset network settings
The phone will switch off and restart automatically in a few minutes
Could I ask do you have a recurring goodybag? Which value is that and what date does it recur?