Users can't log in via Membership 2

When a user tries to log in using the M2 page ( /register/?do-login=1 ), he sees an error message:

Request failed, please try again.

If the same user logs in with the standard WP form, it works properly.

My site has support access enabled. You can test with user: test4140 password: test

I hope you can help me figure out what's wrong. Thanks.