Thanks;
I tried to implement the approach in the stack overflow article you suggested, but it made no difference; the gantry error page is still shown.
I tried to add the whole
<IfModule mod_rewrite.c>
RewriteEngine on
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ / [L,QSA]
</IfModule>
Before or after the existing <IfModule mod_rewrite.c> block in the .htaccess; I also tried inserting
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ / [L,QSA]
inside the existing <IfModule mod_rewrite.c> block (after "## End - Custom redirects"); in all three cases, the gantry error page is still shown.