WAN gets IPv6, but no IPv4 - send_packet: No buffer space available

[this.is.tom]

Hello there,
I just got my first OpnSense (DEC850v2).

The problem is that my WAN port only gets an IPv6 address, but no IPv4.
I have waited for 8h to ensure the DHCP lease from my ISP is timed out, I have power cycled the fiber modem and the OpnSense - all without effect.

Something I see in the logs is alot of "2024-03-01T05:02:58   Error   dhclient   send_packet: No buffer space available".

I found this Netgate page: https://docs.netgate.com/pfsense/en/latest/troubleshooting/buffer-space-errors.html?highlight=buffer

The WAN port is assigend to ax0 and I use the SFP module recommended by my ISP: Optik-1GSFPBiDiLX/S.B1312.10.XDL

Does anyone have any idea?

Thanks in advance!

Tom

[EDIT]
I tested the old configuration and used the CAT cable from the media converter. And all of a sudden it works. The WAN int gets IPv4 and IPv6 and routing works.

Does anyone has any idea what this could be? Why does IPv6 arrives on the SFP but no IPv4 and with copper everything works.