TyniMCE and Image assist

I would like to use image assist and imageGalary to insert images into content (body in page eg.)
I use 6.4 drupal and the problem is that "insert" gives a errormessage something like:

line: 86
error: can't replace Focus to controlellement becaus it isn't visible, disabled or a type that cant accept the focus.

(the error was in Dutch so I hope that is is correct translated.)

When disabling TinyMCE it works correctly for the body field.
could'nt find anything about this at the moment

Thanks in advanced
Wim