Downloads fail and do not get unrarred

Forum rules
Help us help you:

Are you using the latest stable version of SABnzbd? Downloads page.
Tell us what system you run SABnzbd on.
Adhere to the forum rules.
Do you experience problems during downloading?
Check your connection in Status and Interface settings window.
Use Test Server in Config > Servers.
We will probably ask you to do a test using only basic settings.
Do you experience problems during repair or unpacking?
Enable +Debug logging in the Status and Interface settings window and share the relevant parts of the log here using [ code ] sections.