Microsoft Office Ribbon Custom Tabs not working

Question

We have a Office (M365 Apps for Enterprise) Environment.

Usually I was deploying simple Ribbon customizations for Word with the "Word.OfficeUI" file under C:\users\bla\AppData\Roaming\Microsoft\Office.

This worked totally fine for Office 2016. Since we upgraded to M365 Apps for Enterprise this does not work any more. Here is a example of my file. It seems something changed in general with the new Office version.

So my questions:

• Can Word.OfficeUI still be used in M365 Apps for Enterprise?

• Any other idea what could be the issue?

    <mso:tabs>
    <mso:tab id="test" label="ABCD" insertBeforeQ="mso:TabInsert">
        <mso:group id="test2" label="ABC FORM" autoScale="true">
            <mso:button idQ="Button" label="A FORM" imageMso="WindowsCascade" onAction="StartForm" visible="true"/>
        </mso:group>
        <mso:group id="TestGroup" label="TEST" autoScale="true">
            <mso:button idQ="TEST123" label="B FORM" imageMso="RmsSendBizcard" onAction="StartForm2" visible="true"/>
        </mso:group>
    </mso:tab>
    <mso:tab idQ="mso:TabWordDesign" visible="false"/>
    <mso:tab idQ="mso:TabBlogPost" visible="false"/>    </mso:tabs>