This usually is commonly seen when you have the PDF open for preview. If its not open, then try closing Adobe first before you run your script again.
Thanks for the reply,
I tried all that, but to no avail ... what i tested now however is to write and export the PDF to a local folder (instead of to a server-folder before) and it seems to work fine ... for those who understand a bit more about Mac/Microsoft-combinations: i think it has to do with a file-(un)locking in the Microsoft-environment that isn't recognised by Mac (or vica versa) ... i am still interested in the solution here (but i'm already happy that it works for me now ;o) )
Thanks for the efforts!