I found a solution: adding an outbound NAT rule in the firewall, using the VPN interface as source and any as destination.
As I said, I'm not very familiar with NATing and routing and I would have expected a routing rule... So I'll have to dig a bit more to understand the difference between a NAT and a routing rule. :-)
As I said, I'm not very familiar with NATing and routing and I would have expected a routing rule... So I'll have to dig a bit more to understand the difference between a NAT and a routing rule. :-)