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Internal failure, maybe due to missing functions like proc_open!

When validating a delivery server, and you get an error that says “Internal failure, maybe due to missing functions like proc_open!”
Most likely the issue is that MailWizz cannot find a valid email template to send the validation email.
To fix this, go to Backend > Settings > Email Templates > Common Email Templates, and press the “Reinstall core templates” button.
Then try validating the delivery server again.