htaccess redirect throws an error: PHP Catchable fatal error: Object of class WP_Error could not be converted to string

They aren’t really connected. It’s just a redirect after all.

The error means that some code in your website wrongly passes a WP_Error object to wp_kses_no_null(). In your position I’d put something like this into this function for debugging:

if ( is_wp_error( $string ) ) {
  var_dump( $string );
  debug_print_backtrace();
}

This way you can see exactly what the error is and where it’s coming from. Maybe it’s the fault of a plugin or theme, or some misconfiguration.