Hi Andreas,
I tested with 15.6.3.1 in an X460-24x
I am able to configure the protocol filter. 
X460-24x.12 # sh protocol filter cdp detailProtocol Name          : cdp
    Protocol Id Type   : snap
    Protocol Id Value  : 0x2000
    Destination Address: 01:00:0c??cc:cc
    Field Offset       :
    Field Value        :
    Field Mask         :
Current State:    OPERATIONAL
Image Selected:   secondary
Image Booted:     secondary
Primary ver:      15.7.2.9
Secondary ver:    15.6.3.1
X460-24x.14 # conf l2pt profile "allowcdp" add protocol filter "cdp"
* X460-24x.15 # sh l2pt profile
  
            Execute the command
  |               Filter the output of the command
    Show only the specified profile
    "allowcdp"
* X460-24x.15 # sh l2pt profile "allowcdp"
Profile Name                      Protocol Filter Name              Action  CoS
--------------------------------  --------------------------------  ------  ---
allowcdp                          cdp                               Tunnel
Please share the following output: 
show configuration detail | include cdp
Have you modify the CDP protocol which was already defined? If so, what changes were made. 
I think if we exclusively configure the destination-address to this protocol again, we can make it work. 
configure protocol filter "cdp" add dest-mac 01:00:0c??cc:cc snap 0x2000 
Please try this and let us know!