Problem pairing again Nexus 5 with Alpine car stereo
The issue was not Android.  (Except perhaps the misleading error message.)  

The problem, it turns out, was that the pairings on the radio were "full".  It seems this radio (CDE-133BT) allows 3 pairings.  Well, I inadvertently renamed my Nexus some point along the way when I also unpaired it from the phone.  Attempting to re-pair without first emptying a free slot on the radio failed.  I do not know what the comms is like for denying a connection based on that, but either the radio sent incorrect information about why the pairing was denied or Android reported an incorrect reason.

The problem is solved - By the way  - the way to empty the bluetooth pairings is NOT in the manual - I watched a YouTube video of someone making pairings and discovered a way to find what the pairings were.

Not sure if this Q/A still belongs here, but maybe it will be helpful for someone with an Alpine CDE-133BT someday.