TYPO3 throws Error after Updating to 11.5.16 and PHP 8.1 (get_class_methods(): Argument #1

After Updating from TYPO3 9.5.x LTS and PHP 7.4 to TYPO3 11.5.16 and PHP 8.1 I am getting this error:

get_class_methods(): Argument #1 ($object_or_class) must be an object or a valid class name, string given

I know it's about my Extension (when I disable it the Error disappears), but the further debug Information I are not helping me:

in /html/typo3/typo3_src-11.5.16/typo3/sysext/extbase/Classes/Mvc/ExtbaseRequestParameters.php line 302

            // todo: this is nonsense! We can detect a non existing method in
            // todo: \TYPO3\CMS\Extbase\Utility\ExtensionUtility::configurePlugin, if necessary.
            // todo: At this point, we want to have a getter for a fixed value.
            $actionMethodName = $this->controllerActionName . 'Action';
            $classMethods = get_class_methods($controllerObjectName);
            if (is_array($classMethods)) {
                foreach ($classMethods as $existingMethodName) {
                    if (strtolower($existingMethodName) === strtolower($actionMethodName)) {
                        $this->controllerActionName = substr($existingMethodName, 0, -6);

I couldn't find anything searching github, stackoverflow or the web. Anyone else had the same problem or knows where this could come from?

Solution

Right after this Post, it came to my Mind to change the PHP Version back to 7.4. With PHP 7.4 I've got a different Error Message ("Class does not exist. Reflection failed.") which leads me to: https://forge.typo3.org/issues/91239

I made the Mistake to Register Plugin With VendorName. Just in case anyone else does it the same way, I've written this answer and would like to keep this Post.