Comment 13 for bug 1799913

Ok I actually made it work. Not sure if this still belongs here, but might as well inform others how I fixed it. Maybe it will help you as well.

Although I had never connected the iPhone to my PC before, I needed to reset the trust settings to get the dialogue option to "trust this device" on the iPhone.

The procedure is quite simple. Go to Settings > General >> Reset >> Reset Location and Privacy. It is also explained here: https://www.guidingtech.com/fix-iphone-trust-this-computer-not-showing-up-issue/

By the way, I then still could not paste files on the iPhone. This might be an Xubuntu specific issue, but I had to install iFuse first. See here: https://askubuntu.com/questions/799414/how-to-move-files-from-ubuntu-to-iphone