PHP 7.3 and WP5.2.3 site shows white screen

Last night my web host made PHP 7.3 the default and my website failed to load, no error page displayed, just a white screen.
Browser Firefox 69.0
Mac OS X 10.14.6

Changing back to PHP 7.2 and the site displays correctly.

With PHP 7.3

Site health says Jetpack detected an error
Debug at jetpack.com displays:

XML-RPC is not responding correctly ( 500 )

It looks like XML-RPC is not responding correctly. Please make sure XML-RPC is turned on for your site and is set up to respond to all content types. You can test this yourself by running the following command from the command line:

curl -A ‘Jetpack by WordPress.com’ -d ‘<methodCall><methodName>demo.sayHello</methodName></methodCall>’ https://theboxplace.co.uk/xmlrpc.php

The answer should be:

<?xml version=”1.0″ encoding=”UTF-8″?>
<methodResponse>
<params>
<param>
<value>
<string>Hello!</string>
</value>
</param>
</params>
</methodResponse>

I don’t really understand any of that! Is there anything I can do of a non-technical nature?

Thanks

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