row_to_external() should refert to the ftp interface. Try to write it in to a sequential file. And check if there is any thing unexpected. Later try to ftp the same file from command prompt and check if you find any difficulty in that.
Impossible doesn't mean 'it is not possible' actually means... 'NOBODY HAS DONE IT SO FAR'
What derivations are you using in the Transformer? Does this error occur in every row? Use the debugger or stage tracing to figure out what's happening.
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