This is the same shortcoming as it was in the desktop version! You had recognised it as such and had graciously provided a workaround by providing an option to “Ignore TLS certificate errors”.
I’d be very happy if I can get a “valid certificate” via Let’s Encrypt but, like others, I self-host my instance of Nextcloud and use a Dynamic IP Update Service. Let’s Encrypt doesn’t play nicely with this. Hence, a certificate cannot be generated.
You’ve created a great program, Laurent. Thank-you very much. If I could just get it working on my Android devices, it would be a Killer!