That was unfortunately no help.
I rebuilt the server, got a new Access Key, but still cannot connect. As before the PCs not rebuilt connect fine, and I can access over the internet. I tried to Sync the library from the PC that cannot connect, and it accepted the Access key and prompted me for my username and password (never got this far when trying to add the library). The Action Window stated "Analyzing files..." then crashed Media Center. I tried again and this time, as before it read "Analyzing Files". When I move the cursor over screen text it changes to garbage text, for example under Action Window, every choice reads "JRICP" and the left reads "RootItemBackground", etc.; basically crashes.
I have released and renewed my IP address, ensured the router is correct, reinstalled the network driver. Any more ideas? I am at a loss. For now, I "restored" a backup of the library from the server to at least have the library files present, but I lose desired functionality this way and would rather use a single library.