json_encode error

After latest update of this plugin my website stopped working. To eliminate all other possible causes I made fresh WP installation with just CF7 and CF7MSF.

When trying to submit step and go to the next one nothing happens and in dev console I can find this:

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<b>Warning</b>: json_encode() expects parameter 2 to be long, string given in <b>/autoinstalator/wordpress3/wp-content/plugins/contact-form-7-multi-step-module/contact-form-7-multi-step-module.php</b> on line <b>159</b><br />
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<b>Warning</b>: Cannot modify header information – headers already sent by (output started at /autoinstalator/wordpress3/wp-content/plugins/contact-form-7-multi-step-module/contact-form-7-multi-step-module.php:159) in <b>/autoinstalator/wordpress3/wp-content/plugins/contact-form-7-multi-step-module/contact-form-7-multi-step-module.php</b> on line <b>159</b><br />
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<b>Warning</b>: json_encode() expects parameter 2 to be long, string given in <b>/autoinstalator/wordpress3/wp-content/plugins/contact-form-7-multi-step-module/contact-form-7-multi-step-module.php</b> on line <b>159</b><br />
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<b>Warning</b>: Cannot modify header information – headers already sent by (output started at /autoinstalator/wordpress3/wp-content/plugins/contact-form-7-multi-step-module/contact-form-7-multi-step-module.php:159) in <b>/autoinstalator/wordpress3/wp-content/plugins/contact-form-7-multi-step-module/contact-form-7-multi-step-module.php</b> on line <b>159</b><br />
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<b>Warning</b>: json_encode() expects parameter 2 to be long, string given in <b>/autoinstalator/wordpress3/wp-content/plugins/contact-form-7-multi-step-module/contact-form-7-multi-step-module.php</b> on line <b>159</b><br />
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<b>Warning</b>: Cannot modify header information – headers already sent by (output started at /autoinstalator/wordpress3/wp-content/plugins/contact-form-7-multi-step-module/contact-form-7-multi-step-module.php:159) in <b>/autoinstalator/wordpress3/wp-content/plugins/contact-form-7-multi-step-module/contact-form-7-multi-step-module.php</b> on line <b>159</b><br />
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<b>Warning</b>: Cannot modify header information – headers already sent by (output started at /autoinstalator/wordpress3/wp-content/plugins/contact-form-7-multi-step-module/contact-form-7-multi-step-module.php:159) in <b>/autoinstalator/wordpress3/wp-includes/rest-api/class-wp-rest-server.php</b> on line <b>1266</b><br />
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<b>Warning</b>: Cannot modify header information – headers already sent by (output started at /autoinstalator/wordpress3/wp-content/plugins/contact-form-7-multi-step-module/contact-form-7-multi-step-module.php:159) in <b>/autoinstalator/wordpress3/wp-includes/rest-api.php</b> on line <b>551</b><br />
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<b>Warning</b>: Cannot modify header information – headers already sent by (output started at /autoinstalator/wordpress3/wp-content/plugins/contact-form-7-multi-step-module/contact-form-7-multi-step-module.php:159) in <b>/autoinstalator/wordpress3/wp-includes/rest-api.php</b> on line <b>552</b><br />
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<b>Warning</b>: Cannot modify header information – headers already sent by (output started at /autoinstalator/wordpress3/wp-content/plugins/contact-form-7-multi-step-module/contact-form-7-multi-step-module.php:159) in <b>/autoinstalator/wordpress3/wp-includes/rest-api.php</b> on line <b>553</b><br />
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<b>Warning</b>: Cannot modify header information – headers already sent by (output started at /autoinstalator/wordpress3/wp-content/plugins/contact-form-7-multi-step-module/contact-form-7-multi-step-module.php:159) in <b>/autoinstalator/wordpress3/wp-includes/rest-api.php</b> on line <b>554</b><br />
{“into”:”#wpcf7-f8-p11-o1″,”status”:”mail_sent”,”message”:”Twoja wiadomo\u015b\u0107 zosta\u0142a wys\u0142ana. Dzi\u0119kujemy!”}

Any clues what may be the issue and how to fix it?

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