As a first guess, I think this means the record you are importing doesn't match the EDI structure you are using. I ran into this when I mapped EDI records earlier this year: each record must match the structure *exactly*, or it won't work.
Since the error is on line 1, column 1, you might open the file in a non-destructive text editor like Notepad++ and see if you can spot the issue. Sometimes third-party programs that process EDI records add their own header; another possibility is that you are importing a record with the EDI envelope into a structure that expects the envelope (or vice versa).