Sorcol

Thanks @esmi, this worked for me too!
I changed the code for a native WP gallery in my post from [gallery ids="134,135,136"]
to
[gallery ids="134,135,136 link="file"]
and now Fancybox is working perfectly.

I suspect my page redirect problem was caused by inappropriate use of wp_redirect. I was using the code posted here to conditionally redirect to a page’s first child. I used the code within a loop, which worked fine locally but not on the live site.

In the end I created a new page template for pages I want to redirect. The template consists of the following code:

<?php /* Template Name: Redirect Page */ ?>
<?php if (have_posts()) {
  while (have_posts()) {
    the_post();
    $pagekids = get_pages("child_of=".$post->ID."&sort_column=menu_order");
    $firstchild = $pagekids[0];
    wp_redirect(get_permalink($firstchild->ID));
  }
} ?>

The Login/out problem and white screens are fixed!
There were some empty lines after the closing php tag in the custom theme’s functions.php file – I deleted these and can now log in/out. No more white screens either.
Redirect problem still to be resolved…

@songdogtech Thank-you for the very prompt response and the references. I’ll go through them in detail and report back – hopefully I have just missed a step in the transfer.

@krama757 Thanks for that…very useful tip for hiding the textbox.

Sorry! I’ve uploaded the code to Pastebin:
https://pastebin.com/zCHwAitw (page-posts.php extract)
https://pastebin.com/FHn8HATs (sidebar.php extract)