Openmaint send HTTP GET with Accept text/xml header. geoserver 2.16.2 responses with HTTP 406.
Please change text/xml to application/xml to be compliant with newer versions of geoserver.
As you will have read here https://www.cmdbuild.org/en/download/latest-version the software requirements of CMDBuild / openMAINT require to use Geoserver 2.10.1.