When I submit an entry into a form, it redirects

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

I have the same problem with this migrated form:

My problem was resolved by changing this:
go into your admin panel
go to plugins
choose FS contact form
click on the advanced tab
find: Enable an HTML anchor tag on the form POST URL.
and UNCHECK it
your form will now redirect to your page and not to the form

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";

Tried almost everything listed above to no avail. I have turned off almost all of my plugins as well just in case, but no joy there either. I can also say that all other forms on the site are working properly with the same setup, so I’m a little mistified.

Problem form is here : https://www.shadowboxlive.org/community/donation-requests

Working form example here : https://www.shadowboxlive.org/volunteering

I have also pulled the extra php calls on the problem page as well just to check, but nothing there either. Same result no matter what I do. Thanks for all your work – this plugin is the bomb