I don’t know if this is related, but I can’t use F-Droid on Android 6 since it does not contain the Let’s Encrypt cert without rooting the device, and it seems like F-Droid has stopped using the cross-signed version or maybe it’s Let’s Encrypt itself has stopped cross-signing? This happened a few months ago.
I’d wager the root CA certificates on the device are expired since it hasn’t been updated in ages. So it can’t trust certs signed with the new root CAs.
I don’t know if this is related, but I can’t use F-Droid on Android 6 since it does not contain the Let’s Encrypt cert without rooting the device, and it seems like F-Droid has stopped using the cross-signed version or maybe it’s Let’s Encrypt itself has stopped cross-signing? This happened a few months ago.
I’d wager the root CA certificates on the device are expired since it hasn’t been updated in ages. So it can’t trust certs signed with the new root CAs.