When I do apt-get upgrade I do not get the the Maintenance Release 19.1.20190822.
My apt setting is
deb http://download.opensuse.org/repositories/server:/eGroupWare/Debian_9.0/ /
As written in the 19.1 docu: Debian and Ubuntu require to call apt install egroupware-docker to do the 19.1 upgrade.
apt install egroupware-docker
Please read the docu in our wiki, as 19.1 is not just a maintenance release, if you come from 17.1!