Ivan, David, thanks a lot! It works now.
I've created more tests and wrote a huge, detailed, now deleted mail, but in the end it was this hint that brought me to the right track:
On 06/16/2016 05:07 PM, Ivan Markin wrote:
Yeah, now I recall that on some systems with AppArmor/SELinux or on OpenBSD tor as other daemons has restictions applied on the paths accessible for them.
I use the tor project's debian package on the dev machine and it has some AppArmor profiles. After simply adding
/srv/hstest/hstest.socket rw,
to the file /etc/apparmor.d/local/system_tor and reloading AppArmor it works.
Relieved, Johannes