Thumbnail links to galleries hanging, captions not updating

@bob Jefferson, as a test you have to try deactivating ALL plugins and setting a WP default theme, deleting all caches.
If it works well, then reactivate one by one, each time deleting cache and look if and when the error occur to individuate who is the culprit.

Actually, I have discovered that I have a more serious problem. None of my image links are working sitewide. This is an example of the error messages I’m getting:

XMLHttpRequest cannot load https://www.facebook.com/portobello.online. Origin https://porty.org.uk is not allowed by Access-Control-Allow-Origin.

To be more precise, the image links will open in a new tab or window, just not in the same window. I appreciate that this is no longer a NGG issue but has anyone come across this problem before?

Yes, all of your errors (cache and cookies too) have nothing to do with Nextgen, and maybe not even with WordPress.
To debug with WordPress, the advice that I gave you above is always valid, for installed plugins or script in the template.
The Access Control Allow Origin can happens when you are trying to make a JSON request across a domain. Maybe this?
But for that you should ask in an appropriate forum.

Yeah, thanks for your help anyway.