Excel 2013 :: Formatting Of Entire Worksheet Changes Unexpectedly

In Excel 2013 x64 (EN; CZ locale) I have this funny bug. I work on a large vba project and sometimes when I open it, every cell in every workbook that had default formatting now has this numberformat (shown as "Accounting")

"_-* #,##0.00 [$Kč-405]_-;-* #,##0.00 [$Kč-405]_-;_-* ""-""?? [$Kč-405]_-;_-@_-"

its seems that this formatting is assigned to styles --> Normal and it just messes up everything (pivots, slicers...) and cannot(!) be undone.

I have made some routines to check for this error on workbook.open and workbook.close and I also have file versioning. I check for the error regularly on every worksheet change, but it never comes up, nor does it whenever I close the workbook, so Im having hard time detecting when it occurs.

Sometimes when I try to open the workbook its just all messed up. When I go trough the versions, couple of them back still has the error which means it was already saved with it.

All I could figure out so far is that it sometimes happens when I try to copy some cell and paste it elsewhere (but later it works fine)

I'm 99.9% sure that my code is not causing it by accident or purpose. Now I just found the problem on different workbook that might have been opened at the same time. If you're interested, have a look here [URL] ......