Cannot login network user to Open Directory from client Mac.

I run a Mac Mini with OS X Mavericks Server (10.9.1) and an iMac client with Mavericks 10.9.1.

Open Directory is setup on the MacMini server, iMac is joined to the server (users & groups, login options, server with green light, i.e. connected).

When I reboot the iMac client I cannot login to any network user.

What can I do?

Many thanks in advance for your help.

Best regards,

Joe