Hey Nicolaas,
Welcome back to the forums and thanks for reaching out 🙂
If you could, please send over your System Information as a private reply. Also, Easy WP SMTP is a great plugin to help ensure emails are delivered properly.
I see you checked ‘Yes’ to both reverting back to the default theme and deactivating all plugins other than ours as outlined in the Testing for conflicts guide, but can you please confirm this? Lastly, does your email offer the option to display the QR Code image (a message similar to: “Display all images in this email?”) or does it not display at all?
Keep me posted and have a great evening. Cheers!