Error-Messages on gallerysites

Hello,

I have the pro and plus version.

Since one week I get the following errors on the sites, where I use the nextgen plugin.

Notice: Uninitialized string offset: 0 in /var/www/web180/html/merkesdir/wp-content/plugins/wordpress-seo/frontend/class-twitter.php on line 476

Notice: Trying to get property of non-object in /var/www/web180/html/merkesdir/wp-includes/link-template.php on line 315

Notice: Trying to get property of non-object in /var/www/web180/html/merkesdir/wp-includes/link-template.php on line 329

It’s only on the sites with the plugin. When I deactivate the plugin, I get no errors.

Here you find the site.
https://www.merkesdir.de/portfolio/frauenportraets-farbig/