TYPO3 - ext:form not working when using it in gridelements - v8

Question

I have gridelements (the dev branch for 8.x.x) with 8LTS (+ fsc) and ext:form brought to work. Both seem to work for themselves. However, if I place a form element in a gridelement container ... I get the following error message when I want to send the form:

Oops, an error occurred!
The given string was not appended with a valid HMAC.

Anyone an idea what causes this? The other CE elements seem to work in the grid.

the Grid Configuration for all * CE is allowed in the defined container:

Name: Column #1
Column number: 10
Allowed CE: *
Allowed Grids: *

And in Log:

Core: Exception handler (WEB): Uncaught TYPO3 Exception: #1320830018: The given string was
not appended with a valid HMAC. | TYPO3\CMS\Extbase\Security\Exception\InvalidHashException  
thrown in file /is/htdocs/www/typo3_src-8.7.4/typo3/sysext/extbase/Classes/Security/Cryptography/HashService.php  
in line 94.  
Requested URL: .../contact/?tx_form_formframework%5Baction%5D=perform&tx_form_formframework%5Bcontroller%5D=FormFrontend&cHash=c81ac6ed1b41sdfgsdg71fsdfb44584e7

Debug shows:

TYPO3\CMS\Extbase\Security\Exception\InvalidHashException thrown in file
/is/htdocs/.../www/typo3_src-8.7.4/typo3/sysext/extbase/Classes/Security/Cryptography/HashService.php in line 94.

19 TYPO3\CMS\Extbase\Security\Cryptography\HashService::validateAndStripHmac("a:1:}ac4293effb593e4b063cbfcc6fc2c1e2c0c650a1")

/is/htdocs/.../www/typo3_src-8.7.4/typo3/sysext/extbase/Classes/Mvc/Controller/MvcPropertyMappingConfigurationService.php:

00124:         }
00125: 

00126:         $serializedTrustedProperties = $this->hashService->validateAndStripHmac($trustedPropertiesToken);

00127:         $trustedProperties = unserialize($serializedTrustedProperties);
00128:         foreach ($trustedProperties as $propertyName => $propertyConfiguration) {

...