mautic.CRITICAL:Malformed UTF-8 characters

Your software
My Mautic version is: 2.16.1
My PHP version is:5.6

Your problem
My problem is: Because of this error, no email will be sent out.

These errors are showing in the log:
[2020-05-29 09:10:34] mautic.CRITICAL: Uncaught PHP Exception InvalidArgumentException: “The string ,f��&a�U�"���0U{,f��a{�f��/d��4�3n!0�2���alzdhacdh=�f��������{�f�dh&a�U�4,��avzgzW21�dc. tg2�edevjoavz�2.0�{�f��avzf�/k��>r��{,f��&a�W2 ,��Wada}} is not a serialized array.” at /home/patent/public_html/mautic/app/bundles/CoreBundle/Helper/ClickthroughHelper.php line 42
[2020-05-29 14:42:23] mautic.CRITICAL: Uncaught PHP Exception InvalidArgumentException: “Malformed UTF-8 characters, possibly incorrectly encoded” at /home/patent/public_html/mautic/vendor/symfony/http-foundation/JsonResponse.php line 164 {“exception”:"[object] (InvalidArgumentException(code: 0): Malformed UTF-8 characters, possibly incorrectly encoded at /home/patent/public_html/mautic/vendor/symfony/http-foundation/JsonResponse.php:164)"}

Steps I have tried to fix the problem:
This type of error was reported earlier but no stable solutions available. Hence starting new topic.
I tried every thing like cleaning up email content, user emails etc, trying to debug code etc. The email used to send was cloned, not newly created/pasted. Your help on how to check/handle this kind of errors will help a lot.