Hi Philipp,
thanks for your answer. You are right. I didn't used the express file dialog VI. I chnaged it to the express Vi but it still doesn't work. I found out that it's not related to the number of files but it depend rather to the length of the file names.
I tried to open 1020 files with filenames that have about 20 - 30 characters. I only able to open about 760 files with the express file dialog VI. After that I've changed all filenames just to number from 1 to 1020 and I was able to open all files.
It seems that the lengthes of the filename strings are the limiting factor. Strange and not quite usefull, isn't it?
Cheers.
Ronny