I am reporting a similar (as it seems) issue when I visit the Resend page of a webform.
It occurs with Claro and Gin themes. When I try with eg. the Bootstrap admin theme, the issue is not present.
These are the errors I get:
1. Only file JavaScript assets can be optimized. in Drupal\Core\Asset\JsOptimizer->optimize() (line 31 of /var/www/html/docroot/core/lib/Drupal/Core/Asset/JsOptimizer.php).
2. Symfony\Component\HttpKernel\Exception\BadRequestHttpException: Invalid filename. in Drupal\system\Controller\AssetControllerBase->getGroup() (line 224 of /var/www/html/docroot/core/modules/system/src/Controller/AssetControllerBase.php).
gpapadas → created an issue.
I am experiencing an issue that is quite the same. In one particular webform, while I am on the 2nd step I click Next to go the 3rd, I am redirected to the 1st step, and I get an error saying that the required fields of the first step are empty. This happens randomly. The behavior is the same as I was on the 2nd step and clicked the refresh button from the browser and then Resend.
@daniel_arend did you manage to solve this?