A 'Client Import' is just writing to the cell_info file, and a failure is usually a hostname resolution issue
I recommend that you do a 'nslookup' of the client name and IP address from teh command line of the cell manager, and a 'nslookup' of the cell manager hostname and IP address to see where it is not resolving.
You don't say what the OS type is of your cell manager, but, one way around this is to just edit the cell_info file to add the client manually. You can use one of the other lines in this file as a template, just change the hostname field
Your version of Data Protector is out of support, so you can't open a case with the response center