The connection failed because the network is unreachable?
Recently, I started getting this error for my iCloud account and also my Google account. What happens is that I will see the exclamation point icon next to the account in the Calendar list. I click it, and it says:
The server responded with an error.
The connection to account "iCloud" failed because the network is unreachable.
Same problem for Google. If I quit and relaunch Calendar, the calendars will refresh correctly, but as soon as the first scheduled refresh happens, they go into this failed mode.
This renders Calendar useless, because I can't get any updates for those accounts.
Interestingly, my work's Exchange account has no problems at all. It's just the two CalDAV accounts.
MacBook Pro, OS X Mountain Lion (10.8.2), Mid-2009, 4 GB RAM, 500 GB HD