[Users] Some URLs will not open in viewer

[Claws mail user]

In Claws mail v. 3.13.0 some URLs in HTML emails will open a Web page
in the viewer and some won't. This is using the Fancy plugin with the
option "Open in viewer (remote content is enabled)" selected.
Everything else is enabled: display images, execute java script,execute
java applets, render objects using plugins. I'm not sure I can see any
common thread amongst the links that will work with the viewer and those
that won't. If you right click on a link which won't display in the
viewer and choose "Open in browser" this pretty much always seems to
work. Is there a reason for this or a configuration option which will
fix it? Many thanks