ActiveX server conflicts when running excel macro

I have built a simple GUI that simply runs a macro from a Excel sheet.

ExcelApp = actxserver('Excel.Application');
ExcelApp.Workbooks.Open(filename);
ExcelApp.Run('Run_Cases');

When I run the macro from Excel, it tried to get another object (unisim) but returned the following error:

"Run-Time Error 429: ActiveX Component Can't Create Object"

The macro runs fine on Excel alone (not accessing from MATLAB).

Could you please suggest how to solve this? I searched a bit and tried to re-register the MATLAB ActiveX server, but doesn't seem to work.

Thanks, Bob