Even when you are connecting from same version of GUI to same version of Cell Manager and you are getting the following pop up message..
WARNING: You are attempting to connect with your cell console to a Cell Manager of an older version. This combination is not supported.\n\nHowever, you may use this combination during the time you are upgrading your Data Protector environment for tasks, which do not alter the cell configuration. These tasks include starting backups, monitoring sessions, answering mount prompts and previewing reports.\n\nYou must not use this combination for altering the cell environment for example by adding or changing clients, users, devices or backup specifications. Such an operation leads to unpredictable results including potential loss of cell configuration information. Please use the cell console on a client of the same release level instead.
The likely cause is that the "-cs A.xx.xx" entry is missing from the cell_info file on the cell manager, where A.xx.xx represents the version you are on. If that is the case the cell_info can be edited to include the missing string.