Hi,
"I assume that you are creating a new file, then calculating an MD5 as a checksum. Try to use the file path out of the write characters to file as the input to the MD5 function to enforce data flow."
I was checking the MD5 of an existing file, not creating a new one. First I checked it with the MD5 ComputeHash.vi and once the VI was finished with execution I tried to open it with the Open/create/replace File.vi which produced the error code 8. Two other guys tried to figure out as well what's going on, but we got nowhere.
However, we made an MD5.dll with C and that's working just fine so no problems anymore. Thanks for your help.