I get almost the same error on Sametime 8.5.2 enviroment.
[26/10/11 14:15:23:913 CEST] 00000024 PacketSwitchR E Error = 503
[26/10/11 14:15:23:914 CEST] 00000024 PacketSwitchR E REGISTER URI NOT Successful : <sips:PacketSwitchwebchatSTMSNode1@webchat.customer.com>
Googling for answer lead to this post and also to the same TECHNOTE You found. I guess that is our only answer.