Thanks for the replies and suggestions, guys. I don't have any anti-virus app installed but there is Knox running, of course, so some security stuff. I don't know why that should be killing it but I can't see anything to confirm. I'm running Android 11 on my Note 20 so I think I'm pretty up to date. I haven't encountered this type of issue on any app before that I can remember.
Knox is probably not the culprit. I found that the
build script for Electrum is still using Android SDK 29.0.3, while Android 11 has API 30 which would require SDK version 30 too. I think what's happening is that since the Electrum app wasn't built for Android 11, it's not working on Android 11 devices. If other people here confirm this then you can try submitting a Github issue on their repository to use the newer SDK version.