Sorry, I'm new to add-ons on my 320L, so this might be me, but when launching the interface I get the same 403 error as above:
403: Forbidden
Unauthorized IP Address.
Either disable the IP address whitelist or add your address to it.
If you're editing settings.json, see the 'rpc-whitelist' and 'rpc-whitelist-enabled' entries.
If you're still using ACLs, use a whitelist instead. See the transmission-daemon manpage for details.
I have checked under Configure and the rpc whitelist settings are already set as "rpc-whitelist": *.*.*.* and "rpc-whitelist-enabled": true, and I have installed v1.13 but it still won't launch
The link it is trying to open it at is https://[ip_of_NAS]:9091/?pass=admin