Issues OS X Sierra - Version 12

For a couple of days now, I can't connect any remote host from my MBP with OSX Sierra, no error occurs, the connection simply doesn't start.
I can't connect from a windows PC to my Mac either, here I get an "unknown error".

Both directions worked for me until last week - no update or new software were installed, this occured from one moment to the other.

I uninstalled TV12, tried an older version - same issues. Also tried with firewall turned off, but nothing helped.

Any ideas?

EDIT: when in the same LAN, I can connect from PC to MAC using the IP4-Adress of the MAC. But connct from Mac to PC through the IP doesn't work - very strange...