[Plugin: WP Smush.it] Dreaded : '…must be in the content directory…' is back

After updating to 1.50 and trying to smush an image in the media library I am getting the dreaded

/srv/www/vhosts/mydomain.com/subdomains/img/httpdocs/mediathek/2011/10/LPDC-Rafa-03.jpg must be within the content directory (/srv/www/vhosts/mydomain.com/subdomains/img/httpdocs/mediathek/)

After downgrading to 1.41 everything is back to normal and the error message is gone.

https://www.ads-software.com/extend/plugins/wp-smushit/