TYPO3 backend form module gives error when creating forms

Question

I'm currently running TYPO3 version 8.7.18 and running a site_package that was made by using the site package builder: link

Whenever I try to add a form by using the default backend forms module I get the default message Oops, an error occurred! along with that the following error

TYPO3\CMS\Form\Domain\Configuration\FormDefinition\Converters\AddHmacDataConverter not found

Searching for the issue I get several hits on the Gitlab but these do not provide any solution.

Along with this setup, I've added the forms module as a static include in my top-level template.

Answer 1

do you have a composer installation or an extension manager build up?

If a class is not found you may need to rebuild the autoloader information.

Answer 2

My error turned out to be caused by a corrupt installation. I've had to upload the file AddHmacDataConverter.php in the following path /public_html/typo3/sysext/form/Classes/Domain/Configuration/FormDefinition/Converters. A very weird error indeed.

For those that came here with the same error, make sure to check that folder. Apart from that add the news as a static inclusion in your top domain. You could also follow @Bernd Wilke's idea which s to disable the extension, clear all system caches (and additionally remove typo3/temp/autoload and then active the extension again.