There is a router connected to my Safepipe´s public interface. Which router ports must be open in order for VPN Clients to work with SafePipe?

The answer depends on your Safepipe and VPN Client software versions:
For Safepipe and VPN Client software version 1.x, the following ports must be open: ICMP (ping), UDP port 500 (IKE) and IP protocol 50 (ESP).
For Safepipe and VPN Client software version 2.x, the following ports must be open: UDP ports 30295 and 30296, ICMP (ping) (used if the VPN Client does not get a reply from the UDP connection), as well as SMTP if using e-mail notification.
Please note that ICMP (ping) is occasionally blocked by firewalls (make sure it is not, especially if using VPN Client version 1.x).
You can check your Safepipe´s software version by clicking the "About" button on Safepipe´s startup screen.