Error message using PHP 7.4 and CF6

We have a significant amount of development in CF6, and because it is stable we decided not to redevelop in V7.
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Recently our site administrator decided to upgrade the PHP version on the site to 7.4 because of some security alerts and also to keep the site up to date.
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Following this, some of our CF6 forms failed with the following error:

Sorry sent too early... , the error message received is this:
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I DOMXPath::query():  Invalid expression in selector >> body
<< in /home/mysite/public_html/libraries/cegcore2/vendors/emogrifier/Emogrifier.php on line 371
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When reverted to PHP 7.2, the error message does not appear.
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Is there a solution to this, or do we have to remain on 7.2 as long as we continue with CF6?
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Thanks
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Tim

Hi Tim,
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I found another thread here where Healyhatman suggests that you "Turn off the advanced template option in the email action."
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Bob

Thanks Bob,
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Unfortunately that option is cot checked in any emails in the application. So at this stage I have a workaround though it is not entirely satisfactory because of the need to stay on an old version of php.
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Cheers
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Tim