Today when I tried to login on Communicator I got the following message:
“Microsoft communicator cannot sing you in.
Either date and time settings are incorrect, or the digital certificate file is not valid or installed on your computer. If date and time are correct, see your network administrator to verify that your digital certificate file is valid and installed”
To fix MS Communicator CA certificate not valid do the following: Spotlight > Keychain > search for your company CA certificate > double click on it > trust > always trust.