I found the problems finally:
First: libapache2-mod-proxy-uwsgi is a suggested package, however, for
running Radicale in the in Debian suggested way it is a necessary package.
At the beginning I haven't realized that this was missing as I could
enable proxy-uwsgi (link in sites-enabled did exist). I found it through
desperately trying anything.
Second: I changed the filesystem_folder in /etc/radicale/config. But the
default value (/var/lib/radicale/collections) is also anchored in
/etc/uwsgi/apps-available/radicale.ini and as home directory of user
radicale. Propably this all needs to changed to have it working. I
changed filesystem_folder in /etc/radicale/config back to the default
value for now and after that it finally worked.
I will file a bug report for this package.
Flo
