Same problem here. Both on decks and memrise. Also new problems with review sessions that fail to load began after failed bulk adding
Some observations on this bulk adding mistake.
It so happens that memrise doesn’t allow you to add words (at least in bulk) that are exact copies of that are already in your database. It doesn’t matter if they are in the same level or if they are not actually in any level at all, as long as they are in the database they can not be added again. Now this particular bulk adding mistake doesn’t add words to the level but it still adds at least some words (maybe all, haven’t checked that) to the database. So once you have tried and got this mistake the next time you try to add the same bulk of words it doesn’t necessarily happen for the same reason, it simply happens because the words actually were added to the database (though not to the level) and now can’t be added for the second time.
I got that mistake and then I tried to add my words in smaller bulks and still failed. But then I read about they being actually added to the database, so I deleted them from there (one by one, very tedious process) and after that I tried to add my words again in much smaller bulks (10 words at a time) as an experiment and it worked.