There is an odd bug which I found when I selected all the fields on an opportunity record in an advanced find and then tried to export the records to excel.
What made this error annoying was the download log file was greyed out, so it was difficult to investigate.
The reason for the error is because there are two fields with the same Display name, in this case it was two fields with the name Proposed Solution.
To resolve the error you need to rename one of the fields and you will be able to export from Excel again.
You could also not select the duplicate fields and this will work around the error.