I have also experienced this problem; I could manage combine load the files from same folder in a CSV format. Once saved backs to excel, I had the same error. 

I also load separately upload the files as quires and then was able to combine by admen with no error is report.

I also tested reducing the date set in files i.e. columns and rows and problem did resolve!

Has any on been able combine and load an folder with excel files and if get error, find the acetal error in data which is stopping the data capture