There are a few posts talking about this bug.
To fix it:
cd /usr/local/hestia/web/fm/backend/Services/Session/Adapters/
mv SessionStorage.php SessionStorage.php.ori
curl -fssLm20 https://raw.githubusercontent.com/sahsanu/hestiacp/90e175baed239cccab8761dbfdc693bc9642c066/install/deb/filemanager/filegator/backend/Services/Session/Adapters/SessionStorage.php -o SessionStorage.php