Thank you for your fast help.
Tap click worked after I copied the configuration file and added the option. (I had to create the "xorg.conf.d" folder first.) But I lost my Xfce panels. I could only use right-click on the desktop in order to get any menus. Therefore I navigated to the panels configuration. After that my panels came back. It said I need to save the panels after logging out or I would not have them when logging in again.
I found an option to save the panels on logout but each time I logged in I got the error that the panels aren't available. When I then selected to enable the panels I got an error telling me that they can not be changed as a normal user because they run in kiosk mode.
I could overcome this problem by logging out and deleting all files in "/home/username/.cache/sessions" but afterwards I couldn't adapt the panel settings any more.
On top of that the system needed an unusual long time to boot and log-out or log-in. (I had to wait for at least 30 seconds before anything happened when I select either option.) So I suspect there was something failing again but I had no clue how to narrow it down. After all I made a totally fresh installation just about an hour earlier.
Therefore I did the third or even fourth installation this day and this time selected GNOME 3 instead of Xfce. There everything worked out of the box but it is slow – even if version 3.20 does actually need fewer resources than older GNOME 3 versions. So I am again uncertain how to best proceed. Since the Acer Extensa 5220 can't take more than 2 GB RAM it might be better to use Xfce but if I can't get it working properly and keep having ridiculous login- and logout-delays the performance gain might be more than leveled by other issues that slow me down and annoy me. And I didn't even mention the odd behavior that during typing in the LUKS pass phrase I get messages that produce several lines with unfinished pass phrases in the middle. Fortunately, this is more an optical problem since I can keep typing and the functionality is not hindered by the additional output. It just feels very odd that the output is creating copies of what I am typing mixed with error messages I still need to document. (The pass phrase isn't shown. It's just the stars that indicate the length of what I am typing.)
I guess I will give Xfce an other try when I have more time to tinker with it.
[update]:
The ridiculous login times and messages during boot did come up on GNOME 3 also after the first boot. I found a
tip how to resolve this but both options failed to resolve my issue. If anything the boot is even slower now.