Getting Invalid Format in Import File during import for an .xlsx file

Hi,

We just switched from CRM 2011 to CRM 2016 (8.0.1.79), big step. 

In the old CRM we had a process doing

• export contacts (enabling for re-import), exports to .xml
• import in a self build PHP tool that adds data in an already existing column, then export it again
• re-import the file in CRM

this worked flawlessly

Now with the new CRM, it exports to .xlsx. We changed our self build PHP tool, but during CRM import, CRM gives an "Invalid Format in Import File during import" error.

I also did a test with LibeOffice - Calc, opened the CRM export file, did save a copy and tried to import the copy into CRM, same error message.

What magic is CRM import expecting? What is not correct about the import file? I hope someone here can give me some insight.

kind regards

Ido