ExtensionService::getPluginNameByAction() must be of the type string, null given

[devxpoint]

Hello, I use TYPO3 10 with powermail 8.4.2
Created a simple form on a news detail page and get an error after submit:
Argument 1 passed to TYPO3\CMS\Extbase\Service\ExtensionService::getPluginNameByAction() must be of the type string, null given,
called in /.../typo3/sysext/extbase/Classes/Mvc/Web/Routing/UriBuilder.php on line 611
Put it on a single page -> same error

I have other forms on the same website, these are working. :)