It saves now. This has to do with the timing of making the messages available for translation and the time the job to do this is actually processed. These jobs do have priority in processing, but it can take a few minutes. Can you confirm that you tried to translate relatively quickly after the page was marked for translation?

(In reply to comment #1)

Can you confirm that you tried to translate relatively quickly
after the page was marked for translation?

Yes indeed, quite right away after marking the page for translation (as I usually do). Thanks for the explanation Siebrand, it makes sense.

(Random idea: maybe the error message could mention something along the lines of “Échec de l'enregistrement de la traduction : Try waiting for a few moments before attempting to translate (Unknown error: "tpt-unknown-page")”?)

Not working example http://commons.wikimedia.org/wiki/Template:Cleanup_image

(Using Google Chrome on Windows 7 x64.)

Edit: (works now) seems the bot had not started (after 2h) to create a default ./en subpage.

Just encounter same issue when translating this link https://meta.wikimedia.org/w/index.php?title=Special:Translate&group=page-User%3AMCruz+%28WMF%29%2FSandbox%2FWriting+Contests+video+subtitles&language=zh&action=page&filter=!translated

And after I read the discussion here, I tried again, and now I can save now. Thanks to you, volunteers on Phabricator!