Thanks CRo and BlackJack! As it turns out, under Debian (4.9.0-8-amd64 #1 SMP Debian 4.9.144-3 (2019-02-02) x86_64 GNU/Linux)
$ sudo ./roonserver-installer-linuxx64.sh
does not set the correct permissions/ownerships. One has to switch user to root and then just do
$ ./roonserver-installer-linuxx64.sh
Now the server starts fine.