Cannot save document translation

Steps to reproduce:

• Enable multilingual on your wiki with the following settings: 
• Create a new page "test" under Sandbox
• Save some content on the original language (the document needs to more versions in history)
• Go to edit mode and open the translation document
• Add some content and try to save

Result: you cannot save the page because you get the warning box 

Expected result: the translation document gets saved.