We have had trouble with expressions in connections (for files and database) quite often. Sometimes delayed validation provides a fix, sometimes it keeps returning every now and then. As far as I know it is also related to some timing issue, that is why it may not occur on one machine (workstation or server) and seems almost persistent on another machine. If the run does not end properly the file 'connection' may remain open and you cannot access its content or even see that it has some content. I suggest looking for a dangling process and be sure that everything has finished running before you try to open this CSV file.