I had this problem with my subscriptions until earlier this morning. It appears that clearing the app’s cache fixed it for me. If you go to mlem’s settings tab, you’ll find it on the bottom of the General settings page. Tap the rectangle and then return to the feeds tab. In my case, the articles loaded without an error.
I don't have that option, and I've since realised that I can't seem to access TestFlight at all now. It only asks for a code which I don't have, and don't remember how I obtained in Mlem's case.