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Without seeing the code, this is difficult to determine. Be sure to probe the filename that you are writing to. Also, if the file is open, I believe that you will get this error, so be sure it isn't being opened by something else (you could have two "read from Spreadsheets" on top of each other) or something like that. Also, try using Highlight execution and probes, or stepping to troubleshoot the issue. In highlight Execution, you will see the data go into the read function, and if the error occurs there, you can slide the error dialog out the way to see the source of the error.
Good luck