Plesk-Certificate not valid?

[aendgraend]

Hi,

we try setting up our new Webserver using Plesk to log onto.
My Admin-Colleage can log in successfully but i get the Message that the Security-Certificate fo Plesk was only valid until March 2nd 2007 and can be insecure maybe?
When i try logging onto the Server through Plesk i get the Login-Page displayed ever and ever again, and cannot see the Plesk Interface on the Server.

To make things more weird i can sometimes log in, but then i can only make a few Clicks in the Plesk Interface and then I'm getting dropped out again.

Does somebody have a Solution or Workaround?

Thanks in Advance

Andreas