After taking care of the XMPP server, I installed an XMPP client on the Android phone of missis Mees. No worries! But the Pinephone, well that's a completely different story.
First try: Gajim
The obvious choice it Gajim. I read stories of Pinephone users successfully using this app. So I installed it and it worked the first time. But the second time the app stalled. The thirth time it started, but it gave a server error. And when it finally worked, I could not select some functions: the scaling was not right as the program is not responsive. My conclusion: not suitable for the Pinephone
Second try: Dino
Maybe, if I could reach the buttons, it could have worked. Again: not a responsive application. Verdict: not suitable for the Pinephone.
Thirth try: Kaidan
It advertises itself as a modern, responsive app. But it is also not a finished product, as I use version 0.7.0. But for now, it seems to work fine. Kaidan it is!