Just upgraded iCloud to 4.0.1, and now receive message: "You can't sign in because of a server error".

Hello

I have found that this kind of error is usually related to the icloud application not being able to address the network on specific ports. Check to see that:

1. Firewall rules are not stopping communications
2. Anti-Virus / Anti-Malware programs are stopping execution
3. Network stack is in order.

Items 1 and 2 are generally the easiest to check, Item 3 requires checking WINSOCK PROVIDERS to ensure remnants of old TCP or UDP sessions are not "stuck". A reset of the WINSOCK stack generally clears up remaining problems. [ netsh winsock reset ]

Hope this helps