A few questions:

First, I know it's a holiday weekend, but did you put in a ticket? If not, go ahead and do so.

Second, when you log into vPanel and look at the Features --> Devices command, do you see an entry for the line that's failing?

Third, have you tried (for what it's worth--possibly nothing) changing your 'preferred' server in vPanel? Features --> Preferences --> Network Preferences
That page shows which server you're connected to, which may not correlate with your 'preferred' server.

Fourth, have you tried using the SIP credentials with a softphone for the account that's not ringing in?

Fifth, have you tried intentionally taking your 'misbehaving' line down and seeing if/how the failover function works (e.g. to your cell phone)? Do you still get an 'all circuits busy' message, does it forward the call, or is there another issue? (Alternative to taking your line down, have you tried turning on call forwarding in vPanel and seeing how that behaves?)

I hate to hit you with a ton of questions, but I'm trying to think of different things to try that might point out the source of your issue...

FWIW, I'm registered to central, and I just made a successful inbound call.