I have just marked a page for translation on Meta-Wiki.
Markers have been added but translations units (page in Translations namespace) and /en translation subpage have not been created, so we cannot translate the page (Special:Translate redirects to recent additions fixed 20 min later).
The “mark” action has well been logged.
Actually, in the same time, another user has marked another page without any issue.
So the issue may be related with the fact the page has been imported (so maybe not a regression as I firstly thought).
As a separate issue, "Title does not correspond to a translatable message" error should be in the interface language...
As a separate issue, "Title does not correspond to a translatable message" error should be in the interface language...
It seems that the translation actually got saved despite the error.
And in any case making a dumb edit and remarking the page for translation has solved the issue.
I did not achieve to reproduce, so that was probably just a temporary server overload.
I close this task, sorry for the noise.