I have two Radarr instances, one for 1080P and one for 4K. The are exactly the same other than their subdomain and the content they have. Otherwise they are exactly the same version, same machine etc.
The 4K Radarr syncs fine with NZB360. The 1080P version doesn't.
The only difference would be the amount of content they have - 1080P has about 8000 movies - 4K about 1000.
Is the app timing out when loading the library and crashing NZB360?