Can't provide the full error message right now, but next time I see it I'll post it here.
While the object does indeed exist, it exists under the Keys node in SSMS, not the Contraints node.
This is an unaltered installation of DMX 3.7.3, but was originally upgraded from the old DMX3 (pre-DMXLite) version.
Has anyone come across this before. Getting desparate. It's supposed to be a smooth upgrade from DMXLite to DMX 6 isn't it?