76-D0-64-DA-4F-5B

I have fixed the problem by setting the redirection timeout to 0.

I took a look into file “class-fscf-process.php”. Setting redirect timout to 0 the following code is executed:
wp_redirect( $ctf_redirect_url );

Using a meta refresh value > 0 this meta refresh piece of code is executed:
self::$meta_string = "<meta http-equiv=\"refresh\" content=\"$ctf_redirect_timeout;URL=$ctf_redirect_url\">\n";

I have the same problem with this migrated form:

I have the same problem with a migrated form: https://www.hlc-hoexter.de/anmeldung-kursprogramm/