Seems like https://phabricator.wikimedia.org/T152208

In T152208: Articles deleted on CX are still considered as "in progress" the translation in progress cannot be dismissed (it's not clear if the article-translation-in-progress is removed after refreshing the page).

The bug reported is easily reproducible (wmf.12 testwiki):

1. User X starts the translation and then discards it
2. User Y attempts to start the translation for the same article (to the same language, of course), gets a greyed out screen with the warning: "This is ongoing translation by user X. ..."

3. User X has nothing in 'In progress'. When attempting to start the translation of the article (the same article for which the previous translation was discarded) and gets "Failed to load the saved translation!"

Re-checked in .wmf15 - the issue is still present.

There is one more implication for the user Y form the previous comment: the article, in fact, is added to the user's list of translations 'In progress'. And the user Y cannot dismiss it. I attached the screen recording: the translation of an article is started by another user, and the article cannot be discarded by any of the users. And no user feedback is provided.

@Etonkovidova is this related to T125959 ?

@Arrbee yes, it's related - I've added this task and T125959: The translation_started_by value is not updated if translation goes through draft -> delete -> draft by new translator steps as subtasks to T202128: Failure to load old translations (published and in-progress) between Dec 17 and Jan 18.
Also, T202128 has the stack trace of error - hopefully it'd be helpful.