In Membership register button is there but it doesn't show

Hi,
I have installed Membership in a website and I have problems with the register buttons: they are there but they doesn't show.
I opened a thread in Community some days ago but didn't have an answered back. There is a screenshot there: https://premium.wpmudev.org/forums/topic/register-button-is-there-but-it-doesnt-show-membership
Thanks in advance,
Xènia