Error on website when logging into admin

Hi, This website is having an issue logging into the Admin. [login to view URL] See details to login: [login to view URL] First one is jointgu johnny123$ then you will access the admin username and password. admin ***** This is where we get the Server Error.I had someone look at it quickly and say Session is not carrying values, hence admin panel is not working properly. Session is not working properly on the server. I spoke to the server and they said "Could you please advise what you mean by "session isn't carrying values"? As mentioned above we simply provide the cPanel hosting platform and cannot make any other changes for you regarding the site or any of its pages/data." Anyone know what this is. Im sure it is small