Just when I pasted in the error text I noticed the 2013 reference in path.
When installing 4 I got a warning that the older version would get overwritten - which I answered with OK (or yes).
Seems that the version 4 installer is not fail safe. Better to abort installation I think.
I uninstalled 4, removed all remaining references to KiCad manually (Program Files (x86)) and reinstalled.
An yes, the files load fine. It was an old version of the file parser that was used which did not know the new grid_origin keyword.
And yes, I had somehow de-checked the layer. pfff should have known that one.
Anyway, thanks, your response sparked the path to success.