String to array header error

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Hey, I went on my site and there was this error

**Warning**: Array to string conversion in **/data/web/virtuals/273341/virtual/www/domains/petrmacholan.cz/wp-includes/pluggable.php** on line **2279**

**Warning**: Cannot modify header information – headers already sent by (output started at /data/web/virtuals/273341/virtual/www/domains/petrmacholan.cz/wp-includes/pluggable.php:2279) in **/data/web/virtuals/273341/virtual/www/domains/petrmacholan.cz/wp-admin/includes/misc.php** on line **1416**

**Warning**: Cannot modify header information – headers already sent by (output started at /data/web/virtuals/273341/virtual/www/domains/petrmacholan.cz/wp-includes/pluggable.php:2279) in **/data/web/virtuals/273341/virtual/www/domains/petrmacholan.cz/wp-includes/functions.php** on line **6865**

**Warning**: Cannot modify header information – headers already sent by (output started at /data/web/virtuals/273341/virtual/www/domains/petrmacholan.cz/wp-includes/pluggable.php:2279) in **/data/web/virtuals/273341/virtual/www/domains/petrmacholan.cz/wp-admin/admin-header.php** on line **9**

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Never had this before and I am nowhere near educated enough in this. I have my page built on wordpress and elementor. From what I get, there are multiple stuff fighting over the header information. I read on wordpress official forum that you can set up some Used subset(s) and that will get rid of this error – where can I find that setting?

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