As per email discussion - the windows installer startup.bat generated by NSI is no longer functioning.
Windows exe-installer creates a startup.bat file that does not start. For some reason the batch file must be run from GEOSERVER_HOME, not from GEOSERVER_HOME\bin. I am remembering that change to jre8 had this effect with older Geoserver versions as well.
Adding "cd .." manually fixes the issue:
After fixing the launch file Geoserver starts. Login fails still from some pages, for example from About GeoServer with the familiar error
Problem accessing /geoserver/web/wicket/j_spring_security_check. Reason:
Once logged in from the main page things seem to work. One small glitch:
Go to workspace settings, check Settings-Enabled
-> Enabled box gets covered by the title Primary Contact and can't be unchecked. Tested with Firefox 45.0.2.