An annoying message at best, particularly when it’s obvious there’s nothing wrong with “your network”. I had this symptom on a Windows XP machine I installed CrashPlan 3.4.1 on last night.
The solution was to uninstall CrashPlan, turn on “Show hidden files and folders” in Folder Options of Windows Explorer (what if this doesn’t work?*) and hunt and delete all “CrashPlan” folders in the “C:\Documents and Settings\username\Application Data” and “C:\Documents and Settings\username\Local Settings” (hidden) folders.
I then restarted the computer, installed CrashPlan once more, and all was working.
*Bootnote: Turns out some malware had been messing with the registry of that computer and disabled the GUI for “Show hidden files and folders”, rendering it ineffective. I would just tick the box, hit “Apply” and I would still not see the hidden files and folder. A quick scan with Malwarebytes fixed that.