WebOffice supports ArcGIS Server 10.9.1
Thanks to the new features in ArcMap 10.8.2 it is now technically possible again to install the WebOffice extensions for printing on the same machine as ArcGIS Enterprise 10.9.1. Accordingly, an upgrade to these Long-Term-Support versions of Esri products is recommended.
WebOffice 10.9 SP2 is the last release on the latest available ArcGIS version. For further information please refer to the Deprecation Notes of the system requirements or the Esri Product Life Cycle.
In case you have any questions, the WebOffice team will be happy to help you via our support platform!