If you have a prefilled webinar form setup using a single webinar, and find that it is redirecting to a 500 error (red shield), check if the webinar ID to see if it is still valid, as if it is invalid, the API call will fail and cause the prefilled form to redirect to the red error shield.
If you have a webinar form with a multi webinar field setup, if a webinar ID has become invalid, a contact will be redirected to the 500 error red shield page if they have selected that option (so it may seem like random submissions are being redirected to the error page, rather than all of them as it would be with a single webinar form). In this instance, you will need to check the webinar ID's to ensure they are still valid, if you find one or more that are invalid, you will need to remove the multi webinar field and re create it with valid webinar ID's.
If you have checked the above and cannot see any erroneous webinar ID's, please contact your CSM or the Technical Support team, including a description of the issue, the URL for the campaign in which the landing page is found, and any screenshots of the error you may have (that including the URL of the error page)