Some Clients Receive Disfigured E-Mail

Your software Mautic 4.0.1
My PHP version is : 7.4.24
My MySQL/MariaDB version is (delete as applicable): MySQL/MariaDB version 10.2.40-MariaDB

Updating/Installing Errors
I am (delete as applicable): Installing / Updating
Upgrading/installing via (delete as applicable) : Web / Command Line

These errors are showing in the installer :

These errors are showing in the Mautic log :

Your problem
My problem is :
Some of my clients are receiving a disfigured e-mail with a gray box which takes up about 90% of the screens with the e-mail squashed on the left-hand side.

Steps I have tried to fix the problem: We have sent e-mails to various accounts as a test and the e-mails were rendered correctly. We have asked one client to forward the e-mail to us who received a disfigured e-mail. It appears that the e-mail is converted to an image since I could copy it in Thunderbird as an image and paste it into LibreOffice Writer. See the screenshots on how the e-mail appears in Mautic and how the client receives it.

Have you tried to recreate the email in question using the new builder?