I don’t know this app, so I’m just guessing… What I would check are the app’s permissions in the settings. I could imagine it needs access to location. But as I said, just a wild guess
I contacted the Call a Bike support and they replied that the App tries to contact Google Maps directly after start.
I assume that fails and lets the app crash. Not sure, if that is a problem of the app or microg.
Of course, they don’t officially support microg but the support told me they forwarded my report to the developers. Let’s see if anything happens.
FYI: in the meantime, an update for the Call a Bike has been published and with this current version the app does not crash anymore.
Map is still not working, of course, but renting bikes by manually tying in the bike number should work.