Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
So I've uninstalled the Office Compatibility pack and ran ALL Office removal Fix-Its from Office 2003 through 2010 and then it was finally possible to install Office 2010.
Office 2013 removal tool
https://support.microsoft.com/de-de/kb/2822317
Office 2003 - 2010 removal tools