Hey @Nick87, now it’s my turn to run into trouble.
Everything was fine until it stopped working and I get the same message you got:
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 tried everything and nothing works. Tweaking the configuration, removing everything and installing it back. Nothing.
The weird part is that even when the daemon is not installed I still get the same error, as if the problem was something not related to Transmission at all. How did you open the port in your firewall?
Any thoughts are welcome at this point, thanks.