This is a new bug when using another URL than admin.
You need to make changes to the following file:
\vendor\magento\module-backend\App\Action\Plugin\Authentication.php
See the code changes over at https://github.com/magento/magento2/issues/33749#issuecomment-908145941
This issue will be resolved in 2.4.4.
Change the \vendor\magento\module-backend\App\Action\Plugin\Authentication.php
Change the function _redirectIfNeededAfterLogin in this file. replace with the following code:
protected function _redirectIfNeededAfterLogin(\Magento\Framework\App\RequestInterface $request)
{
$requestUri = null;
// Checks, whether secret key is required for admin access or request uri is explicitly set
if ($this->_url->useSecretKey()) {
// $requestParts = explode('/', trim($request->getRequestUri(), '/'), 3);
// $baseUrlPath = trim(parse_url($this->backendUrl->getBaseUrl(), PHP_URL_PATH), '/');
// $routeIndex = empty($baseUrlPath) ? 0 : 1;
// $requestUri = $this->_url->getUrl($requestParts[$routeIndex]);
$requestParts = strpos(trim($request->getRequestUri(),'/'), $request->getFrontName()) === 0 ?
explode('/', trim($request->getRequestUri(), '/'), 4) :
explode('/', trim($request->getRequestUri(), '/'), 3);
if (($key = array_search($request->getFrontName(), $requestParts)) !== false) {
unset($requestParts[$key]);
}
$requestParams = $request->getParams();
unset($requestParams['key'], $requestParams['form_key']);
$requestUri = $this->_url->getUrl(implode('/', $requestParts), $requestParams);
} elseif ($request) {
$requestUri = $request->getRequestUri();
}
if (!$requestUri) {
return false;
}
$this->_response->setRedirect($requestUri);
$this->_actionFlag->set('', \Magento\Framework\App\ActionInterface::FLAG_NO_DISPATCH, true);
return true;
}