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Chronofroms Mulipage Back-Button| Element Link | PHP Error

i searched the forum but didnt found a similar question regarding my problem:
I created a Back-Button similar to the Demo-Form.
When i press the button following PHP-Error appears (I've shortened the path for better reading):
Warning: call_user_func_array() expects parameter 1 to be a valid callback, class 'Chronoforms6' does not have a method
'cf6_preview_trim_posts' in ../wp-includes/class-wp-hook.php on line 288​
​Warning: Cannot modify header information - headers already sent by (output started at ../wp-includes/class-wp-hook.php:288) in ../wp-includes/pluggable.php on line 1216
I also attached the config of the Back-Link.
Can you give me a hint what could cause this problem or how to get closer to a solution?
Do not hesitate to ask for further information if needed.
Thanks for your help,