Thank you for your post.
I found a post reporting the same error: Vaadin error from users browser when loading a WebDirect Solution
If you could provide further information:
-Does the issue occur in other Browsers on the issue machine?
-Does the issue occur on all machines? (All Operating Systems)
-Can you get an idea of how many people might be trying to run the Export Field Contents at one time? (several people hit a button or trigger the script at once?)
-Any further information you can provide on what's happening before, during, after, etc...
Testing and Development has been unable to reproduce the error and was asking for a file and steps to reproduce. I am sending a message to your inbox with instructions to send us your file and steps to reproduce if possible.