Hi @Felix_Witte and other affected users in this thread.
Thank you for bringing this issue to our attention.
By all indications, Vodafone Germany has implemented blanket IPv6 with DS-lite on residential accounts. Please see the master thread here: Vodafone Germany: No port forwarding for home accounts. DS Lite Tunnelled IPv4 address that doesn't do port forwarding. [NOT Resolved for Most Users; See Posts. Roon Investigating]. The team is actively investigating a workaround and we’ll continue to post updates there. It’s an unfortunate situation but one we hope to resolve in the near future.
Note that the issue does not extend to all Vodafone subsidiaries or networks outside Germany, as far as we know. I strongly recommend users still continue to troubleshoot even if Vodafone is their provider, as we may be able to find a temporary workaround or identify an issue unrelated to carrier-grade NAT/IPv6.
Thank you for your patience and believe me, I relate to your frustration as I’ve bumped up against carrier-grade NAT myself. Some internet service providers offer public IPv4 addresses for residential accounts; some even offer it for free. Others do not offer it all, and resort to a more basic implementation of DS-Lite that does not support port forwarding for any use. Certain users will be frustratingly arrested if their internet service provider is not willing to work with them. The timeline is unclear for a resolution from the Roon team side; it may take longer than any of us want, and it may never work for every user. That said, rest assured we are aware of the quandary you’re in at this time and are working to make ARC a workable app for as many users as possible, given the limitations and variations inherent to internet service providers.