Just checked and all German NordVPN servers claim to support P2P and SOCKS5, so this is not the issue.
Did you configure the SOCKS proxy as well for “Tracker” and “DHT”? Aside that everything looks like it should, matching the official docs: https://nordvpn.com/de/tutorials/socks5/deluge/
Your second pic is port forwarding in the router?
And when you disconnect from NordVPN, seeding works again? (SOCKS proxy should be possible to leave active as this is independent from the VPN tunnel)
In case I would contact NordVPN support about this:
Mention you use Debian (to make them asking/thinking irrelevant distractions) with the default OpenVPN APT package (which is what DietPi-Software installs) and their official de521.nordvpn.com.udp.ovpn config.
Most important is then indeed that seeing (via Deluge) works without VPN connection active and broken with VPN active, regardless of SOCKS being enabled or not (following https://nordvpn.com/de/tutorials/socks5/deluge/), so it is clear that port forwarding in the router is enabled and the network in general works as expected.
I am still not sure if I understood the NordVPN-side port forwarding issue that you and also others report, especially since I found way more reports about successful seeding through NordVPN tunnel. And 2P2 logically inherits that both sides can reach each others, so it would not make sense if NordVPN claimed 2P2 functionality but then would not forward the requests. 