No port forwarding for connections from OPNsense itself / fix Nextcloud backup

[micmon]

I use OPNsense to connect my LAN to the internet using a PPPoE connection to the DSL modem. Inside my LAN I have a Nextcloud server running behind some Let's Encrypt SSL Proxy. OPNsense sets up DynDNS for mydomain.com and port-forwards 80/443 to the Nextcloud Server/SSL-Proxy. So, if I connect o https://mydomain.com/ from a client insinde my LAN or from the outside, connections get translated to Nextcloud by OPNsense just fine.

Problem is that Nextcloud backup does not work this way, because OPNsense also tries to connect to https://mydomain.com/ as well and the port forward rule (interface=WAN, proto=TCP, source=*, destination=nextcloud) is not triggered because the connection does not originate from WAN. However there is no loopback or similar which could be used as the interface to make this work.

Any idea how to fix this? Using the internal LAN IP for the Nextcloud machine won't work because of the SSL proxy which needs the FQDN.