I had installed it on the DEV box which has the source files for DNN farmework, but not the DMX source code. When I load the solution again, it automatically includes the UI files for Labelcontrol and CustomFielsControl. I guess what happens is, as I do not have the source code behind files for these 2 controls and these 2 controls are not in the DLL, so I get the error.
But never mind, I was easily able to get around by just removing these 2 GUI files from the solutiion & the Bring2Mind folder. What I should do actually at the beginning is to separate the DEV source compiled version from the installed version.