Error loggin in from web panel SSL error

Hi all, i’ve just eredited the administration of an UCS school server. for me is first experience whith this server system an with linux systems too…
Today i was trying to logging in from web page to add a new user but the system gives me this error:
Error message: error:1416F086:SSL routines:tls_process_server_certificate:certificate verify failed (certificate has expired)
trying to find solutions i read something about restart slapd service but before do i need some informations…

This may solve problem? or i’ve to update some SSL certicate?
can i restart this service while users are working or it’s better wait ?

please help me