I am running CiviCRM 5.10.0 on Drupal, and I see the following scenario happening, very consistently:
- go to one of my event registration forms: https://www.terranova.tamera.org/de/civicrm/event/register?reset=1&id=1994
- submit the form although many required fields are still missing.
- Form refreshes with all validation errors. Notice that the url has changed and lost its 'id' param, for the current event.
- complete the registration form correctly and try to submit again.
- Receive an error saying: Sorry, due to an error, we are unable to fulfill... Could not find valid value for id