strip_tags() in media.php error message

Hello,

I am getting this error message on the front page for a featured post on https://handkaes.eu:

strip_tags() expects parameter 1 to be string, array given in ... wp-includes/media.php on line 758

The line is:

'alt'	=> trim(strip_tags( get_post_meta($attachment_id, '_wp_attachment_image_alt', true) )), // Use Alt field first
		);

What do I need to change?

Many thanks,
Johannes