So just to clarify; if i install nzb360 v10.8.3 everything works great and NZBGet works fantastic.

But somewhere after that (version) the URL/configuration string for NZBGet was broken and no longer works.

v10.8.3 is starting to get old now and I'm missing out on some of your new features you got. Hence i just thought i'd shed light o this in hopes it's a simple fix.

The working NZBGet URL (in v10.8.3) is:
https://user:password@domain/nzbget/- The user:pass is for the http authentication in place.
Then the NZBGet Username and NZBGet Password are left blank because i don't have one; but i tried populating these too without any success.

I also tried different variations of the user/pass by just setting to domain to https://domain/nzbget/ and putting the user/pass combo in the entries below without any luck either.

The error message i get back using the newer versions of nzb360 when I press 'Test Connection' is: "Failed to connect to primary address Try again?"

From my end of things, my configuration isn't changing; just your amazing app is being upgraded. On my end I Just have a hostname with a bunch of /paths each mapping to different apps (Sonarr, CouchPotato, etc). I have some password protection on the NGinx server that serves this data as a proxy.

Any advice you have would be awesome! :)
Yeah, v10.9 changed the way I handled settings and connected to services. The underlying code to connect didn't change much, but they way it was entered into the UI did quite a bit. If you PMd me your NZBget connection details I could debug and figure out the right way of entering them in for your setup post v10.9.