Steps to reproduce:

• Have multiple languages on yout eZPlatform instance
• Create a ContentType with a required and translatable ezimage or ezbinaryfile field
• Create a ContentObject of the previous ContentType in one language
• Add a new translation to the ContentType, basing it on the existing language
• The image/file field should be pre-filled, do not touch it
• Publish the translation

You do not get any visible error beside the "Connection error 400" banner but the js logs tell you that "Value for required field definition '<thefield>' with language '<newlanguage>' is empty"