Excel (2013) Power Query: [Expression.Error]: The key did not match any rows in the table - what files(s) is source of this error

I have tens of Excel files with similar table (means: same name, same headers, different content). I'd like to merge all data from these tables to one big table for future processing.

I did it many times before using these steps (I have non-English version, so strings could be slightly different):

1. Power Query > From file (in Load external data Section) > From folder
2. In dialog window with list of Excel files in folder Combine and load
3. In Window Combine files select the wanted table name.

Now I usually get one big merged table. But for one set I get [Expression.Error]: The key did not match any rows in the table

In Query editor window there is a few files loaded OK and then it ends on whole row filled with Error.

This could be easily walked around by using checkbox "Skip files with errors" in step 3, but I'd like to know what file(s) are causing this error(s).

How to find it (them)?

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