I need some further information to help you with this. The type of Linux client, the version, the kernel version, etc.
I'd also like to know the serial number of the filer in question.
I'd also like to see packet captures attempting the access to the cifs share.
I'd also like to know the name of the cifs share.
I'd also like to know the ip address of the client attempting to access the cifs share as well as what IP address on the filer it's attempting to access.
There's a lot of variables here that could be causing problems so I apologize about all the questions.