WordPress search problem Warning: addcslashes() expects parameter 1

Hello!

I am getting the following error message after using the wordpress/woocomerce search:

Warning: addcslashes() expects parameter 1 to be string, array given in /www/htdocs/XXXXXXXXX/wp-includes/wp-db.php on line 1197

Warning: addcslashes() expects parameter 1 to be string, array given in /www/htdocs/XXXXXXXXX/wp-includes/wp-db.php on line 1197

Warning: addcslashes() expects parameter 1 to be string, array given in /www/htdocs/XXXXXXXXX/wp-includes/wp-db.php on line 1197

Warning: addcslashes() expects parameter 1 to be string, array given in /www/htdocs/XXXXXXXXX/wp-includes/wp-db.php on line 1197

Warning: Cannot modify header information – headers already sent by (output started at /www/htdocs/XXXXXXXXX/wp-includes/wp-db.php:1197) in /www/htdocs/XXXXXXXXX/wp-includes/pluggable.php on line 1178

I already followed the instructions her https://codex.www.ads-software.com/FAQ_Troubleshooting#How_do_I_solve_the_Headers_already_sent_warning_problem.3F

I couldn’t find any php-Problems. I also changed the pluggable.php and the wp-db.php files with some fresh WordPress-files. This also didn’t help.

I added following code at the beginning of the pluggable.php
<?php ob_start(); ?>
This fixed the last warning.

Now I am getting only this warning:

Warning: addcslashes() expects parameter 1 to be string, array given in /www/htdocs/XXXXXXXXX/wp-includes/wp-db.php on line 1197

Warning: addcslashes() expects parameter 1 to be string, array given in /www/htdocs/XXXXXXXXX/wp-includes/wp-db.php on line 1197

In line 1197 in the wp-db.php is the following code:
return addcslashes( $text, '_%\\' );

Any ideas, please?

Thanks in advance!