Excel Online Ribbon not visible on SP2016 xlsx file

[Stefaan De Vreese]

We have a Sharepoint 2016 environment. Something strange happens when we open a XSLX file in Excel Online:

the ribbon is not showing. I have tested this in Chrome and IE11. 

Also this only happens on XSLX files. An older XLS file opens Online and shows the ribbon. 

Docx files also open without issues.

Any help would be appreciated.

[#2]

Do you have a menu under the formula bar?

If so you may click on any item and pin the ribbon with right bottom icon

[#3]

No menu visible.

but i have found the issue. Every DOM element of the ribbon is set to "display: none":

The strange thing is, that this does NOT happen with XLS files.

Any ideas on why this is happening and how to solve?

[#4]

Nope, unfortunately have no idea how to fix.