[TYPO3-seminars] blank page after registration

[Harald Hochwald 1]

Hi,
i'm running typo3 4.7.0,Ameos Formidable 1.1.549
Registration is working, but i get a blank page after the second registration
page.
i set the plugin.tx_seminars_pi1.thankYouAfterRegistrationPID = 321
and the page 321 is only hidden in menu (no user acces....).
There is no php error, no html source, just a blank page.

it look like typo wants to load the registration page again and is not
redirecting to the thankyouafterregistration page.
Is there a third step i didn't configure correct.
i set the extions to 2click registration.
tia
Harald