carsten_lin

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This can be caused by a lot of things. For example a SafeMode problem or a not writable temp directory on the webserver or the mod_security feature from the apache. Therefore it would be helpful to know the exact error message.
In case it is because of the SafeMode: disable the “Organize my uploads into month- and year-based folders” function under Settings, Miscellaneous.

I just checked the media.php in the includes directory.

// as per wp_get_attachment_image_src, but returns an <img> tag
function wp_get_attachment_image($attachment_id, $size='thumbnail', $icon = false) {

	$html = '';
	$image = wp_get_attachment_image_src($attachment_id, $size, $icon);
	if ( $image ) {
		list($src, $width, $height) = $image;
		$hwstring = image_hwstring($width, $height);
		if ( is_array($size) )
			$size = join('x', $size);
		$html = '<img src="'.attribute_escape($src).'" '.$hwstring.'class="attachment-'.attribute_escape($size).'" alt="'.attribute_escape($alt).'" />';

         }

	return $html;
}

it seems to be ok, the alt-attribute should be inserted.
I hope you understand what I mean. Here is a example of my site: https://www.halle-fotos.de/innenstadt/bilder-rund-ums-fahnenmonument-am-hansering/attachment/parkanlage/