An update - I finally got a response from
DCL:
“I've provided these details to our development team and they are currently investigating this issue. In the meantime, they've recommended using the website until this challenge is resolved.”
Even though I had included all the things I tried in my initial contact with them, the fact that they didn’t suggest any other possible fixes but went straight to “use the website” suggests strongly to me that they’ve had other complaints and know the latest version of the app is unstable at best.