Favicon using http instead of https. How do I fix this?

I have recently converted my WordPress site to use an SSL, i.e. HTTPS instead of HTTP.

Everything works apart from the favicon. Whether I load or delete the favicon in Settings > Appearance > Customise > Site Identity > Site Icon, it comes up in the source code as follows, with http instead of https.

<link rel="shortcut icon" href="http://mydomain/wp-content/uploads/favicon.ico" />

Of course, this produces an error in most browsers.

The code that does this is the following extract from header.php.

<?php if(  isset($options['iop_favicon']) && $options['iop_favicon'] != '' ){ ?>
<link rel="shortcut icon" href="<?php echo $options['iop_favicon'];?>" />
<?php }?>

I cannot find a solution to this problem. Please help.