Hi Armin, 

don't worry, this error usually appears when the project is more than 4GB big, as the solution is quite simple:

• Once you have the .iwzip file, rename it as .zip (if you do not see the filename extensions you have to enable them from the Windows control panel).
• Then extract the contents of the file into a folder, you can use a default program like 7zip or select "extract" from the Windows menu.
• Select the contents you have extracted in the folder and recreate the .zip storage (to do this you can use either the Windows default compressor or 7zip).
• Then  rename the file you have just created from .zip to .iwzip, so to import it into version 13 by clicking on the specific button "import" and by selecting the file.

Please let me know if you can solve the issue. Kind regards.