Hello again,
After thinking a bit more (should have done that before), I found the solution: I added an outbound NAT mapping to NAT source IP network 10.60.0.0/16 to the public virtual IP.
The reason is that when I ping 8.8.8.8 using the WAN1 interface, pfSense uses the WAN1 private IP as its source IP.
However, there was no NAT mapping this IP to the public IP.
So, I understand that the pfSense box (always?) uses the WAN interface when accessing the internet.
I wrongly assumed that all traffic originating from the pfSense box would use the localhost 127.0.0.1 IP address as its source IP, which would then be NATted to the specified public IP address.
Thanks for all help.