Your browser is auto filling in the port, likely because it saved the value from when you configured SAB or nzbget since that is a common SSL port for Usenet. Fix the port then you should be able to save the settings.
Sonarr doesn’t allow ports under 1024 unless it is port 80 or 443 to be used, the reason being they are reserved ports for other systems.
Port 563 in particular is blocked by most browsers, but browser auto complete was automatically injecting it and people were losing connectivity to Sonarr so we added a check to prevent that. Unfortunately you’ll need to pick another port in this case.
The error message definitely needs to be improved.
That’s a little odd, because I’m actually using port 563 right now. My Sonarr is allowing port 563 to be used. Maybe an older version allowed it and it just happen to be 563.
I’ve been on the developer channel and bound to port 82 since well before Sonarr was called Sonarr. This change locks out a setting arbitrarily and without full explanation.
This ought be override-able. Provide a warning for those users who might not understand that ports can be reserved.
Either way, for more advanced users, you can manually edit the config file to overcome this issue.
We decided to do this to protect the 99.9% of users that are using a default port and prevent autofilled ports (often from SAB settings) from breaking peoples setups.
I don’t see an override for this setting be useful for most users and as you said if you really want to you can set it in the config.