DCM pulse-with checks are specified in the Data Book as being dependent upon the use of the DLL or DFS portions of the DCM. This allows a higher (and lower) operating frequency for users of the CLKFX, to the exclusion of the DLL outputs.
However, this is not included in the timing analysis, and false warnings are issued.
This problem is fixed in the latest 5.1i Service Pack, available at:
The first service pack containing the fix is 5.1i Service Pack 3.