CSRFTokenValidationException | Checkout saveShippingPayment

Hallo Zusammen,

Kann mir jemand bei einem Problem nach dem Update von 5.1.6 auf 5.2.6 behilflich sein?
Erhalten den folgenden Fehler beim wechseln der Zahlungmethoden (Zur Kasse - Anmelden - Zahlungsmethode ändern).
Es erscheint ein Grauer Overlay und geht nicht weg. Im Browser erhalte ich den JS Fehler "Failed to load resource: the server responded with a status of 400 () " in der Datei “https://www.domain.ch/stage/checkout/saveShippingPayment/sTarget/checkout/sTargetAction/index

Auszug aus dem Errorlog:
 

[2016-10-03 11:34:56] core.ERROR: exception 'Shopware\Components\CSRFTokenValidationException' with message 'The provided X-CSRF-Token is invalid. Please go back, reload the page and try again.' in /home/www/gartenam/htdocs/stage/engine/Shopware/Components/CSRFTokenValidator.php:161 Stack trace:
#0 [internal function]: Shopware\Components\CSRFTokenValidator->checkFrontendTokenValidation(Object(Enlight_Controller_ActionEventArgs))
#1 /home/www/gartenam/htdocs/stage/engine/Library/Enlight/Event/Handler/Default.php(91): call_user_func(Array, Object(Enlight_Controller_ActionEventArgs))
#2 /home/www/gartenam/htdocs/stage/engine/Library/Enlight/Event/EventManager.php(214): Enlight_Event_Handler_Default->execute(Object(Enlight_Controller_ActionEventArgs))
#3 /home/www/gartenam/htdocs/stage/engine/Library/Enlight/Controller/Action.php(143): Enlight_Event_EventManager->notify('Enlight_Control...', Object(Enlight_Controller_ActionEventArgs))
#4 /home/www/gartenam/htdocs/stage/engine/Library/Enlight/Controller/Dispatcher/Default.php(523): Enlight_Controller_Action->dispatch('saveShippingPay...')
#5 /home/www/gartenam/htdocs/stage/engine/Library/Enlight/Controller/Front.php(223): Enlight_Controller_Dispatcher_Default->dispatch(Object(Enlight_Controller_Request_RequestHttp), Object(Enlight_Controller_Response_ResponseHttp))
#6 /home/www/gartenam/htdocs/stage/engine/Shopware/Kernel.php(176): Enlight_Controller_Front->dispatch()
#7 /home/www/gartenam/htdocs/stage/vendor/symfony/http-kernel/HttpCache/HttpCache.php(487): Shopware\Kernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#8 /home/www/gartenam/htdocs/stage/engine/Shopware/Components/HttpCache/AppCache.php(255): Symfony\Component\HttpKernel\HttpCache\HttpCache->forward(Object(Symfony\Component\HttpFoundation\Request), true, NULL)
#9 /home/www/gartenam/htdocs/stage/vendor/symfony/http-kernel/HttpCache/HttpCache.php(258): Shopware\Components\HttpCache\AppCache->forward(Object(Symfony\Component\HttpFoundation\Request), true)
#10 /home/www/gartenam/htdocs/stage/vendor/symfony/http-kernel/HttpCache/HttpCache.php(275): Symfony\Component\HttpKernel\HttpCache\HttpCache->pass(Object(Symfony\Component\HttpFoundation\Request), true)
#11 /home/www/gartenam/htdocs/stage/engine/Shopware/Components/HttpCache/AppCache.php(133): Symfony\Component\HttpKernel\HttpCache\HttpCache->invalidate(Object(Symfony\Component\HttpFoundation\Request), true)
#12 /home/www/gartenam/htdocs/stage/vendor/symfony/http-kernel/HttpCache/HttpCache.php(206): Shopware\Components\HttpCache\AppCache->invalidate(Object(Symfony\Component\HttpFoundation\Request), true)
#13 /home/www/gartenam/htdocs/stage/engine/Shopware/Components/HttpCache/AppCache.php(114): Symfony\Component\HttpKernel\HttpCache\HttpCache->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#14 /home/www/gartenam/htdocs/stage/shopware.php(101): Shopware\Components\HttpCache\AppCache->handle(Object(Symfony\Component\HttpFoundation\Request)) #15 {main} [] {"uid":"e02defa"}

Plugins wurden alle relevanten deaktiviert oder gar deinstalliert.
Shopware Tickets dürfen wir leider dank den SLA nur für Livesysteme schreiben (handelt sich hier um eine Kopie des Shops im Unterordner).

Beste Grüsse, Arya_Svitkona