Access denied for __anonim ERROR after adding JCR content in JPP 6

Issue

We store content into the JCR using the following code:

    Node fileNode;
    Node resNode;

    if (node.hasNode(name)) {
        fileNode = node.getNode(name);
        resNode = fileNode.getNode("jcr:content");
    } else {
        fileNode = node.addNode(name, "nt:file");
        fileNode.addMixin("exo:privilegeable");
        resNode = fileNode.addNode("jcr:content", "nt:resource");
    }

    in = new FileInputStream(file);

    fileNode.setProperty("exo:permissions", defaultPermission);
    resNode.setProperty("jcr:mimeType", mimeType);
    resNode.setProperty("jcr:encoding", "UTF-8");
    resNode.setProperty("jcr:data", in);
    Calendar lastModified = Calendar.getInstance();
    lastModified.setTimeInMillis(file.lastModified());
    resNode.setProperty("jcr:lastModified", lastModified);

    saveSession();

When we try to see the new images in the portal, we can't see them in the beginning. We always get an error like:

"ERROR [exo.jcr.component.webdav.GetCommand] Access denied []:1[]media:1[]media:1[]website:1[]portal.jpg:1 for __anonim: javax.jcr.AccessDeniedException: Access denied 
at org.exoplatform.services.jcr.impl.core.SessionDataManager.readItem(SessionDataManager.java:609) [exo.jcr.component.core-1.15.0-GA.jar:1.15.0-GA]".

We are accessing the image through the JCR REST service, eg: http://server:port/rest/jcr/repository/portal-system/media/media/website/portal.jpg

And it does not matter, if logged in as root or not at all. We have read-access for anonim-user, so that even anonymous users can see the portal.

After several minutes all new images are visible and we don't get the error messages any more.