Theme editing error “Sorry, that file cannot be edited”

Since updating to v4.9 (running on IIS) and also updating the Bento theme and installed plugins I have been unable to update theme pages on my, in development, site.
On attempting to update any theme page there is the error “Sorry, that file cannot be edited.”
I have read about and will adopt a child theme strategy but for now would like to resolve the error.
I have:
Checked permissions for IUSR and network Services
Checked files are not read only
De activated all plug-ins
Activated the TwentySixteen theme (same error message)
Logged in on other browsers and machines in order to eliminate problems with browser or security.
None of this has helped.
As a relative newbie I may be missing the obvious so any help is appreciated.
thanks in advance