Urgent: 500 Error when submitting form

Dear all, i really enjoy using your plugin to register users to my page. Today i ran in a major issue which needs to be fixed asap.
When a user is trying to send the form for registration i get the following error:

jquery.min.js?ver=3.6.1:2 
 POST https://www.der-steuerzahler.de/wp-admin/admin-ajax.php 500
send	@	jquery.min.js?ver=3.6.1:2
ajax	@	jquery.min.js?ver=3.6.1:2
s.ajax.s.ajax	@	jquery-migrate.min.js?ver=3.3.2:2
e	@	front.multi.min.js?ver=1.21.0:1
(anonymous)	@	front.multi.min.js?ver=1.21.0:1
dispatch	@	jquery.min.js?ver=3.6.1:2
y.handle	@	jquery.min.js?ver=3.6.1:2
trigger	@	jquery.min.js?ver=3.6.1:2
(anonymous)	@	jquery.min.js?ver=3.6.1:2
each	@	jquery.min.js?ver=3.6.1:2
each	@	jquery.min.js?ver=3.6.1:2
trigger	@	jquery.min.js?ver=3.6.1:2
s.fn.<computed>	@	jquery-migrate.min.js?ver=3.3.2:2
update_buttons	@	front.multi.min.js?ver=1.21.0:1
handle_click	@	front.multi.min.js?ver=1.21.0:1
(anonymous)	@	front.multi.min.js?ver=1.21.0:1
dispatch	@	jquery.min.js?ver=3.6.1:2
y.handle	@	jquery.min.js?ver=3.6.1:2
trigger	@	jquery.min.js?ver=3.6.1:2
(anonymous)	@	jquery.min.js?ver=3.6.1:2
each	@	jquery.min.js?ver=3.6.1:2
each	@	jquery.min.js?ver=3.6.1:2
trigger	@	jquery.min.js?ver=3.6.1:2
(anonymous)	@	front.multi.min.js?ver=1.21.0:1
dispatch	@	jquery.min.js?ver=3.6.1:2
y.handle	@	jquery.min.js?ver=3.6.1:2

The page I need help with: [log in to see the link]