Since an update from 4.38.0. to 4.40.0 Rocket.chat crashes immediately when opening a channel. There’s a bugreport on Github[1] which indicates that just the version available in F-Droid is affected by that bug. When downgrading to 4.38.0 (F-Droid version) the app works.
Never mind, just saw that there’s already a ticket: Rocket.Chat 4.40.0 crashes when opening a chat (#3076) · Issues · F-Droid / Data · GitLab