The last few version (currently on 0.7.3) I had big problems with downloading. Big files, multiple smaller files, after a while my internet connections (iMac late 2009, Snow Leopard 10.6.

froze up. I could not leave the download unattended. Browsing or using the internet on one of the computers/iOs Pods on my network failed. I could see the download/network speed reduce to zero. Time left rose gradually to a few days. ZabNZBd was not responding, had to force quit the application. After restart of ZabNZBd I got my network access back and could resume the download. Usaually, after, 10-15 minutes the problem returned. I could not count on my mac to complete one single dowload. Very frustrating.
Tried every option in the ZabNZBd setup, nothing worked. Re-installed browser plugins, thought about reinstalling my whole system.
Then I got an idea. I checked the Open in 32-bit mode box on the ZabNZB application. Restarted the application and everything is as smooth as butter again.
Now I can leave the downloads unattended again