Sven, how is the content of the barcode itself generated? If the values are generated as part of the script of the form then be sure that the code can be executed on both client and server (from the script window). It sounds like the code may be server-side only at this point.
this was not the solution.
We build the content of the barcode in ABAP. Its a combination of 128C and 128A and its 50 chars long.
There is no script to be executed.