I'm trying to get some transactional email templates working however when I try to preview them, I get redirected to the 'Email Templates' page and get ' An error occurred. The email template can not be opened for preview.'

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Debugging

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I checked var/log/system.log and get the following whenever I try to preview the email template:

 main.INFO: Broken reference: the 'notification.messages' tries to reorder itself towards 'user', but their parents are different: 'header.inner.right' and 'header' respectively. [] []

I also checked the XML files in vendor/magento/module-email and didn't see anything out of the usual.

Any advice would be greatly appreciated!

In case anyone runs into this issue, unlocking the config fixed this for me. The config.php file had multiple settings that were locked so once I reverted this, the previews started to work again.

I'm not sure what specifically caused this however this is what fixed it for me!