So I never managed to get it working in my old NC instance (23 and 24).
Now, after setting up a new instance 25 (due to issues with updating - different thread…) I see that the Rocketchat integration app is still not compatible with NC 24 and newer.
Given that 24 has been out for a while, does that mean the integration is dead?
Nah, devs not updating to every new version of Nextcloud is typical since all it usually requires is changing the version number in a text file every few months (which they forget to do).