Hey, I was having the same issue, running on version 3.55.0.100330. I reached out to Starling support, gave them my error reference and Graphene/Android version. Approx 24 hours later they advised releasing version 3.55.1.100703 which I updated to and everything is working again for me now.
The general vibe I got from my interactions with their support agent is that they seem happy to support GrapheneOS but they cannot guarantee future updates won't break the app, however we should get in touch whenever it does.