Acually, I think it is not a workaround but the solution. ADB might need root-priviledges to access the device on the hardware level. For me it is the same thing (Debian). On my wife’s laptop it works without root-priv. (openSUSE). I guess this is due to different rights settings.
This worked liked a charm! The fun fact is that I already had a .rules file with those attributes (aren’t they the same for htc devices), but it was named 99-android.rules.
Now I wonder which should be the difference between these to files, but nevermind. Thanks for your reply!