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