Safari can’t establish a secure connection to the server

"I have just solved this problem by following this procedure published by phurd99 in the Apple Support Forums:

Open applications
Utiliities
Open Keychain
Click Keychain Access and select Preferences.
Click the ""Reset My Default Keychain""

(...)

http://discussions.apple.com/thread.jspa?threadID=2138178&start=15&tstart=0

This solves the problem but you are left without you precious keychain (a backup is saved).

Then, using the backup and trial and error I discovered that by deleting one specific certificate, the problem is solved:

UNT-USERFirst-Hardware
Intermediate certificate authority
Note: This certificate is signed by an untrusted issuer"