UPDATE: Better info here.
Suddenly, Richard's AC could not open a file on the network, and would hang (rainbow ball of death) when attempting to browse a network resource, such as the Hotel. We noticed it when attempting to open another project in Attribute Manager; since the previous state of that Open dialog was viewing the Onion, hang.
All conventional treatments exhausted, I had to call tech support. They directed me to this knowledge base article, which has to with switching off a cross-platform browsing preference that we don't need anyway.
If you ever see this behavior in AC, here's the fix.
Make sure AC is not running. Open Terminal (Applications : Utilities : Terminal). Paste this text into the window and strike enter:
defaults write com.graphisoft.AC\ 9.0.0\ USA\ v1 InputOutput -dict "DisableCrossPlatformMountingFeatures" "<true/>"
That's it. Quit Terminal and relaunch AC.