you're communicating through the programming port using AS511 protocol. right? The protocol is slightly different for 16-bit and 32-bit S5 CPUs, some address fields in the AS511 header are wider (32-bit).
Your HMI software should query the PLC at the beginning of communication (or have a parameter set manually) and then decide which AS511 version should be used (16-bit or 32-bit addressing). If it doesn't, then you will be never able to communicate to your 135U