PHP error expects parameter 1 to be a valid callback

Try manually resetting your plugins (no Dashboard access required). If that resolves the issue, reactivate each one individually until you find the cause.

If that does not resolve the issue, access your server via SFTP or FTP, or a file manager in your hosting account’s control panel, navigate to /wp-content/themes/ and rename the directory of your currently active theme. This will force the default theme to activate and hopefully rule-out a theme-specific issue (theme functions can interfere like plugins).

Thanks. I think I found the issue – I had edited my theme functions file because I was getting this message:

PHP Notice: The called constructor method for WP_Widget is deprecated since version 4.3.0! Use __construct() instead. in /myserver/public/wp-includes/functions.php on line 3624

with this:

// to change warning for deprecated function
add_filter('deprecated_constructor_trigger_error','_return_false');

Just swapped one error message with another. What to do?

See under “WP_Widget error” here for instructions on how to properly fix the original issue: https://www.ads-software.com/support/topic/read-this-first-wordpress-43-master-list?replies=4#post-7314897

Thanks but it’s not actually a solution to the problem. Anyway, I have ‘hidden’ the error log…

Ah, sorry. There’s a second link there after the explanation which takes you to the solution: https://make.www.ads-software.com/core/2015/07/02/deprecating-php4-style-constructors-in-wordpress-4-3/