I have tested the bulk export import extension on our own OpenCart installation. I have not experienced any issues with importing, even after changing the data in the file. The error leads me to think the OpenCart has issue with either the format of the file, which should be XLSX, or that the formatting of one or more of the columns is incorrect.
I was able to cause the error, but only after intentionally changing the save format of the file.. Changing it back to xls/xlsx allowed it to be imported properly. Be sure that the saved file you are uploading is indeed XLSX. And if that is the case, you will want to check the columns to ensure that they are either numeric, text, or datetime as the original file exported.