The error code 526 is currently appearing for everyone, even if their ARC works, because of the ongoing Roon cloud issues that are mentioned in the blue banner on top of the forum. See:
If you want to configure ARC with port forwarding, it might be best to wait until this is error code 526 is fixed by Roon Labs, so that it doesn’t get in the way. That said, the “MultipleNatFound” error in your diagnostic box is probably a real issue. Search for MultipleNatFound here:
If you are not using two or more routers at home, then the issue might be with your ISP and it may be difficult or impossible to overcome (depending on the ISP).
Generally, it is much easier to forget about port forwarding (and any and all errors that may appear in the diagnostic box) by simply using the Tailscale option to enable ARC access without port forwarding. See here:
And if you are running the RoonServer on Windows, continue here: