Wow, that was quick - many thanks to you @Lakotaubp and @makeixo! Indeed, wiping the caches using twrp as suggested in the linked topic helped: The recovery did not reboot anymore and I was able to complete the installation of UBports.

Unfortunately, I got a black screen afterwards. So I did the following:

removed card tray (was empty anyways, but who knows) downloaded and flashed the (older) SW binaries for Android 7.1 as my device had the newer ones for Android 8 before switched to the "edge" channel

Not sure which of these did the trick, but now UBports boots up just fine.👍