Why is the EXCEL XLSM format no longer a valid ZIP format?

Question

Files saved in Excel as XLSM files are no longer valid ZIP files, which prevents tape editing.

XLSM files saved on or before May 23, 2014 can be renamed .ZIP and edited.

XLSM files saved after May 23, 2014 cannot be renamed .ZIP and edited, but rather they will generate an error message that the file is a damaged archive. Both Windows Explorer and WinZip generate the same error condition, although the error message changes slightly.

Yes, there are macros in the files; but even opening an old file with disabled macros (and VBA) and saving it immediately. How a new name generates a damaged file.

I also tested 2 more machines in our corporate group with the same results, so this is not corruption only at my workstation.

Office Diagnostics does not report problems with EXCEL.

Any thoughts on reasons or solutions?

Update

Let's clarify my test process:

• I will rename the XLSM file saved May 23 to .ZIP; this creates a zipped archive that WinZip and Windows Explorer can open successfully. I will then cancel the rename to create the XLSM file again.
• I open the file above in EXCEL-2007 and do not include macros or VBA . I save this with the new file name as an .xlsm file.
• I rename the file saved in step (2) as a .ZIP file. This file no longer opens in WinZip or Windows Explorer, but generates the error message above.
• I repeat steps 1 to 3 above on the workstations of two other colleagues - exactly the same result.

Update # 2
The problem seems to be related to the files in some way, since saving an empty book since XLSM is still working. I will explore more.

Answer 1

The problem is that one or more sheets are password protected. Unprotect sheets, then save as xlsm, rename to .zip and voila, now the XML structure appears.

Update : In fact, it was an inconsistent password protection setting for the workbook, not for any worksheet.