Event registration form loses 'id' url param and then throws an exception

Question

I am running CiviCRM 5.10.0 on Drupal, and I see the following scenario happening, very consistently:

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

Answer 1

I solved it in a very unusual way - I added this rewrite on .htaccess file:

RewriteEngine on
RewriteCond %{HTTP_HOST} ^www\.terranova.tamera\.org [NC]
RewriteRule ^(.*)$ https://terranova.tamera.org/$1 [L,R=301]

To unify all accesses to use the non-www url. For a reason I cannot yet explain, this completely resolved the issue.