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.
NZB's add fine, but they never start to download - dashboard just says "Idle" even though it's unpaused and I can see a queue of files ready to go. Don't think anything's changed in the config, so not sure what's caused this all of a sudden. I've tried NZB's from multiple sources, and through other downloading clients they work fine.
The error you mention can only occur when you let SABnzbd get an NZB from an indexer.
Try to restart SABnzbd.
Or use the "Repair" button in Status->QueueRepair.