[plt-scheme] MysterX - (make-object mx-browser%) - fails.

I was trying a simple MysterX example:
(require (lib "mysterx.ss" "mysterx"))
(require (lib "class.ss"))
(make-object mx-browser%)
(block-while-browsers)

It gives the following:
make-browser: Can't create browser window

Usually on subsequent executions it hangs.

While this code works fine:
(require (lib "mysterx.ss" "mysterx"))
(define xl (cci/progid "Excel.Application"))
(define wb (com-get-property xl "Workbooks"))
(com-invoke wb "Add")
(com-set-property! xl "Visible" #t)

Any suggestions?