Chiel I'll investigate I'm visiting Hungary next year and had no issues in Prague recently with their PID app. Although I'm generally an offline and paper ticket person wherever I travel. The timetable/route planning really helps.
Having got the logs, and after some tinkering around, can confirm that MAV+ is using Play Integrity API and even with our toggle to block it's usage it just fails. It also doesn't like being behind a VPN either as not being in Hungary and having DAITA turned off on Mullvad caused it to crash straight away. Resolving that enabled it to launch and cycle the names but eventually crash with the Play Integrity warning being thrown.