the problem was solved half an hour later ... I did a ./install -f as suggested and pressed "Y" at the point where I couldn't continue (see log file, last line). Then the setup ran through ...
The installation on a second Raspi shortly after ran directly through. Did you make any changes to the install on uavmatrix.com?
I have almost everything up and running now. Only one point is still missing: VPN access via Zerotier. I installed Zerotier on the Raspi according to the documentation on zerotier.com. I can also select Zerotier in the GUI, but I don't have a field for the Zerotier Network-ID. What else is missing?
Edit: just read this, I am still testing ... https://discuss.uavmatrix.com/d/11277-box-with-you-zerotier-id-doesntt-show/2