Thank you for contacting us with your issue.
I just tested the installer on a new image with RPi3B and was not able to recreate the issue.
The last three lines of yellow text in your webinterface should be:
Processing triggers for fontconfig (2.11.0-6.7) ...
Setting up gsfonts-x11 (0.24) ...
Finished - please click here to reload Page to see Changes
What is your exactly hardware setup?
What user are you referring to? You should not be able to do anything in our plugin before installing the server.