OK
I found that the rewrite was the problem.
I had changed as follows:

RewriteCond %{REQUEST_URI} !\.[a-zA-Z]{2,4}$
to:
RewriteCond %{REQUEST_URI} !\.[a-zA-Z]{2,4}$ [OR]
RewriteCond %{REQUEST_URI} \.(html|htm|php)$ [NC]

I changed it back to basic:
RewriteCond %{REQUEST_URI} !\.[a-zA-Z]{2,4}$

and it works fine.

Not sure if:
RewriteCond %{REQUEST_URI} !\.[a-zA-Z]{2,4}$ [OR]
RewriteCond %{REQUEST_URI} \.(html|htm)$ [NC]

would work. I didn't take the time to find out. Maybe someone else has an answer to that.
Hope this helps someone
John