This is nzbs.org, beta.nzbs.org, nzbsrus, and nzbmatrix.
The only one that worked upon retry was nzbmatrix. I use sabconnect++ in chrome stable channel. The extension is confirmed working at many index sites. These particular failed downloads were either from sickbeard or couch potato, both of which are also working fine (I can find and download things manually with each program, and it works)
In the status screen there is lots of these messages:
2012-05-08 20:37:52,888 WARNING: Empty NZB file 06be0d2d93c34cb27c8e78aa6298f47c.nzb
2012-05-08 20:30:35,255 WARNING: Empty NZB file 09c8f0eee331c5c534991f2c539fa38c.nzb
2012-05-08 20:25:26,744 WARNING: Empty NZB file nzb
2012-05-08 20:25:20,724 WARNING: Empty NZB file index.php!action=getnzb&nzbid=996310&i=40498&h=51a447449872c4a70a1089cabfccebac.nzb
2012-05-08 20:16:35,982 WARNING: Empty NZB file e8502662cc5cb8b43673edda87bd27fb.nzb
2012-05-08 19:55:45,746 WARNING: Empty NZB file index.php!action=getnzb&nzbid=981610&i=40498&h=51a447449872c4a70a1089cabfccebac.nzb
2012-05-08 19:49:51,855 WARNING: Empty NZB file index.php!action=getnzb&nzbid=981610&i=40498&h=51a447449872c4a70a1089cabfccebac.nzb
2012-05-08 19:49:45,046 WARNING: Empty NZB file nzb
2012-05-08 19:49:38,983 WARNING: Empty NZB file index.php!action=getnzb&nzbid=996310&i=40498&h=51a447449872c4a70a1089cabfccebac.nzb
2012-05-08 19:20:23,825 WARNING: Empty NZB file download.php!id=1271145&username=vinister&apikey=76b0988df22117664d7d93717d9e6561&scenename=1.nzb
2012-05-08 19:18:49,200 WARNING: Empty NZB file index.php!action=getnzb&nzbid=980973&i=40498&h=51a447449872c4a70a1089cabfccebac.nzb
2012-05-08 18:53:50,763 WARNING: Empty NZB file index.php!action=getnzb&nzbid=941648&i=40498&h=51a447449872c4a70a1089cabfccebac.nzb
2012-05-08 18:53:11,993 WARNING: Empty NZB file index.php!action=getnzb&nzbid=981610&i=40498&h=51a447449872c4a70a1089cabfccebac.nzb

Thanks for any suggestions!