From the analysis of the presented code snippet, it seems that the problem should not lie in the CZE_DET.OUT files. The import of the old version as well as the new one runs correctly. The "Part" header is irrelevant here, since the data from the CZE_DET.OUT files are taken from line No. 7, i.e. the header is omitted.
In the presented code, the following is highlighted: 
Windows(strShipperName).Activate ' This line does not work, for NO reason!
If, in fact, an error arises on this line then there are two possibilities: either there is no workbook whose name is stored in the strShipperName variable or this workbook is not open. The second possibility is that the window name is not assigned to the strShipperName variable. We should infer that this assignment is in another procedure, which unfortunately we do not see, just as we do not see the assignment to the colAllBuildings collection. Without knowing the rest of the code, it is difficult to answer what should be corrected in the code.

Artik