Consegui cavar um pouco mais e encontrei as seguintes entradas no system.log:
acwebsecagent[257]: Connection : Failed to connect to XXX.XXX.XXX.XXX:80. Code : 60
Jul 29 12:53:47 zzzz.local acwebsecagent[257]: Connection : Caught exception. Code : 60
Jul 29 12:53:47 zzzz.local acwebsecagent[257]: Connection : Failed to connect externally. Code : 60
Jul 29 12:53:48 zzzz.local acwebsecagent[257]: Connection : Failed to connect to XXX.XXX.XXX.XXX. Code : 60
Jul 29 12:53:48 zzzz.local acwebsecagent[257]: Connection : Caught exception. Code : 60
Jul 29 12:53:48 zzzz.local acwebsecagent[257]: Connection : Failed to connect externally. Code : 60
Jul 29 12:53:51 zzzz.local acwebsecagent[257]: Connection : Auth key is not provided or is invalid, applying connection failure policy. CMode : 2 TMode : 1
Acontece que esses erros estavam sendo registrados pelo aplicativo CISCO AnyConnect .
Eu não uso mais esse aplicativo, então o removi e parece (até agora) que o problema foi resolvido.
Se eu ainda precisasse usar este aplicativo, é possível desativar a filtragem da Web Security:
Disabling and Enabling Filters Using Mac OS X
The service password used in this procedure is configured in the Authentication panel of the Web Security profile editor.
Step 1 Launch the Terminal application.
Step 2 Change to the /opt/cisco/anyconnect/bin folder.
Step 3 Disable or enable filtering:
To enable filtering, enter acwebsecagent -enablesvc
To disable filtering, enter acwebsecagent -disablesvc -servicepassword