This happened with both Safari and Chrome
Seconded, Chromium and Firefox alike.
Not sure if this is related, but I feel it might be: Memrise2anki isn’t working, either (I know it’s not Memrise’s responsibility, but it provides some extra data in case it is, indeed, a related issue stemming from a common cause). I’ve found it’s getting error 400 when trying to access preview data with requests similar to this one:
Same just happened to me, the preview session could not be loaded and an error appeared.
Google Chrome, this course
Note: not sure if this is an old or new development but it seems that previews are working for non-completed levels (and for those, requests sent by Memrise2anki seem to work fine).
Edit: no! In fact, only yet unlearned words can be obtained!
Edit2: all’s fine and working as of 10 Feb 2022.
Thanks for reporting this! This is now fixed, but please let us know if you still experience any issues.