WordPress 504 error

Hello @wfpraat

Hope you are doing well.

The URL you mentioned is related to Jetpack CDN which is part of site accelerator feature.

You should contact the plugin developers for better support, this might be a temporary issue and you can disable that feature to fix this. When it’s disabled, the images will be shown from your server.
https://wordpress.org/support/plugin/jetpack/?view=all#new-topic-0

Best Regards
Amin