if "C:\Program Files\SketchUp\SketchUp 2015\ShippedExtensions\su_webtextures.rb" (and according subfolder) is not avail or blocked by a security solution resp. missing user rights, this may result in the described effect too.
install SU again by "right-clicking > run as admin" in connection with the Windows user account which should be used for running SU (i.e. not e.g. an admin account).
ensure, that plugins are not disabled at "Window > Ruby Console" by:
is_disabled = Sketchup.plugins_disabled?
ensure, that JavaScript (not Java) is not blocked by a browser plugin of your security solution.