plugin conflict

Hello,

I’m using wordpress on biz.nf, and i’m getting several times the error bellow :
Warning: mysql_set_charset(): Error executing query in /srv/disk9/2018051/www/iteh-maroc.co.nf/wp-includes/wp-db.php on line 791
Warning: Cannot modify header information – headers already sent by (output started at /srv/disk9/2018051/www/iteh-maroc.co.nf/wp-includes/wp-db.php:791) in /srv/disk9/2018051/www/iteh-maroc.co.nf/wp-includes/pluggable.php on line 1228

After few minutes, the sites run correctly again.

I disactivate one by one some pluguins , and again the error still occuring.

Thanks