Strong passwords – PHP error when saving user

I am getting the following error, when forcing strong passwords:

Warning: Cannot modify header information – headers already sent by (output started at …/plugins/better-wp-security/modules/free/strong-passwords/class-itsec-strong-passwords.php:80) in …/wp-includes/pluggable.php on line 896

For now I disabled this feature.

https://www.ads-software.com/plugins/better-wp-security/