Membership Subscription Form not showing

Hello everybody and thanks for read!

I'm using the Membership plugin and in the register page (and neither on widgets or another page/post) the register form, the register form is not showing at all.

I tried to change the theme to TwentyTwelve. But the problem persists and seems that there is not related with theme compatibility.

The only plugins i have are:
-AJAX Thumbnail Rebuild (unactive)
-Akismet (unactive)
-Membership Premium (active)
-Wordpress SEO by YOAST (active)

When i look at the source code, there is nothing code produced by the shortcode, and of course, nothig is shown.

The Membership Premium Plugin set up is completed:
-Wordpress settings, General: Any visitor can register on your site. MARKED
-Membership Options, at Membership Pages, all actions (register, account, subscription, etc) have their own wordpress page related
-The PayPal Express Gateway is active and the Options Page fullfilled
-There are Subscription Plans asigned to Access Levels, and the 5 test users have their plan related

I don't know wich configuration i could change, everything is set and i tried some configurations.

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UPDATE:

I tried to set a blank page with the shortcode, and the form is showing with all the Subscription Plans, but if i assign that page to "Register" action in the Membership Options, the form not appears anymore until i remove the option from Membership Plugin Options

I made a register1 blank page and a 301 to a register2 page with the shortcode, but i think that there is an issue that must to be solved.

Any Idea?

Thanks