Membership2 - Sidebar Login is not working


the login via sidebar is not working, instead this message appears:

"Request failed, please try again."

when the user/member tries to login or to re-create his pw. I've already checked within the Membership2-Settings pages if anything may block the sidebar-login, but couldn't find anything, nor in WP-settings.

Login via wp-login is working as expected.

Any hint where else to look at?

Thank you and kind regards,