If both HMCM and MMFP restraints were applied simultaneously, that would explain the result; you've restrained the dimer COM to the point 0,0,10 with MMFP GEO SPHERE RCM, as the default values for e.g. XREF are zero if not specified.
It would be best to use just one method, either CONS HMCM alone, or MMFP, but with a PLANAR shape; either would confine the COM to a plane with the given z value.
If you intend to use DOMDEC, you need to use CONS HMCM, as the MMFP restraints are not supported. DOMDEC does support umbrella sampling via the commands described in umbrel.doc and adumb.doc