I have just solved this problem by following this procedure published by phurd99 in the Apple Support Forums:
- Open applications
- Open Keychain
- Click Keychain Access and select Preferences.
- Click the "Reset My Default Keychain"
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:
Intermediate certificate authority
Note: This certificate is signed by an untrusted issuer