TYPO3 Core

Hello,

I have the following setting:
TYPO3 7.6.0, php5.5, a new, upcoming website

On one page (uid=22) there is a form for newsletter registration, done by EXT:form, complete setup included in the plugin itself.
The plugin works as expected, entered values are processed, checks for empty fields and valid email-address work as they should do.

On all pages of the website there is a newsletter form included in the footer.
This has to be included completely by typoscript, as I have to rewrite the actionUri to pid=22 with stdWrap functionality and that is not available when including just the record with the plugin.
The generated actionUri here is now correct (issue #71565), but the form doesn't work. When pressing submit on any page, the user is redirected to the newsletter page with an empty form. No entered values are shown in the input fields, no error messages are printed.
It seems like the submitted values haven't processed at all.
I have checked both forms:
- they have the same fields
- all name values are the same
- they have also the same hashes for arguments and trustedProperties in hidden fields
When I check the request with firebug, I see the same post data for both requests (from the plugin form and from the footer form).

The only explanation I have for this behaviour is the check of the referrer. And as it is not the same page, preccessing the incoming data is aborted.
But I don't know if this is related to form or extbase, but form seems closer to me.