Network TimeMachine backup errors - System keychain problem?
I found an answer on the Ars Technica forum, which helped to get the issue resolved: http://arstechnica.com/civis/viewtopic.php?t=1225417

The file at /Library/Preferences/com.apple.security.plist contained the following text:

&lt;?xml version="1.0" encoding="UTF-8"?&gt;
&lt;!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd"&gt;
&lt;plist version="1.0"&gt;
&lt;dict&gt;
   &lt;key&gt;DLDBSearchList&lt;/key&gt;
   &lt;array/&gt;
   &lt;key&gt;SecItemSynchronizable&lt;/key&gt;
   &lt;true/&gt;
&lt;/dict&gt;
&lt;/plist&gt;


It was necessary to remove the DLDBSearchList key and array from that file:

&lt;?xml version="1.0" encoding="UTF-8"?&gt;
&lt;!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd"&gt;
&lt;plist version="1.0"&gt;
&lt;dict&gt;
   &lt;key&gt;SecItemSynchronizable&lt;/key&gt;
   &lt;true/&gt;
&lt;/dict&gt;
&lt;/plist&gt;


After removing that entry, Time Machine network backups are working normally (as NetAuthSysAgent is now able to locate the correct System keychain item).