This error occurs very likely because test questions include special characters. Try rewriting test questions that do not contain any of the following special characters:

 /   :   "   ;   <   >   #   ?   *   |

Observed Result:  

"An error occurred processing this row" message is observed, the Excel file only loads some questions truncating everything after and including the question containing the special character.
At this point, Blackboard does not have a patch to fix this in development.