Post-mortem
Telia Tunnistus web site at https://tunnistus.telia.fi was served with an SSL Certificate chain that did not validate on Android 13 (or similar, older) operating environment lacking up to date root certificates. The Chrome Web Browser, Android 14 and have recent enough root certificates and did work without problems.
This only affected Telia Tunnistus IdP, not Signicat or other alternatives. Telia has been silent on this, all information is via DVV.
(2026-09-15 11:48) Resolved DVV reports that the issue is solved.
(2026-09-15 07:07) Investigating DVV reports an issue with authenticating using mobile apps on Android 13 or older.