I have an application (Moodle derivative) that uses PHPExcel. Recently, the export of *.xlsx files has started going wrong. If I open the file in Linux's Archive mounter and extract it, and then rezip by using
find -type f | xargs zip ../file.xlsx
it works, but before this, it is gobbledegook in Excel, LibreOffice etc.
I don't know what has changed. Could it be that the server's PHP zip function is doing something wrong?