Fatal error when trying to convert

Error received when trying to convert:

An AJAX HTTP error occurred. HTTP Result Code: 500 Debugging information follows. Path: /batch?id=543373&op=do_nojs&op=do StatusText: error ResponseText: {"status":true,"percentage":"100","message":"Processed 4 out of 4 steps.\u003Cbr \/\u003EExporting CSV..."}

First sign of an error is on the mapping page, when two strips of headers appear.