ModeShape / JCR InvalidItemStateException blocks DMS

Question

In my web application, I have integrated a DMS using ModeShape. Usually, it works perfectly fine. However, from time to time, the application does not allow writing new files anymore due to an exception:

InvalidItemStateException: Node 'folder/filename' is versionable but was not yet persisted via Session.save(), and therefore no version history is available yet

My code for adding a new file to the DMS looks like this.

protected boolean addFile(Session session, String path, String filename, InputStream fileContent, boolean overwriteIfExist, boolean isAbsolutPath) {
    try {
        Node folder=null;
        if(isAbsolutPath) {
            try {
                folder = session.getNode(path);
            } catch (PathNotFoundException e) {
                LOG.error("Path not found in DMS for path "+path,e);
            }
        }
        else {
            Node root = session.getRootNode();
            folder = root.getNode(path);
        }
        if(folder!=null) {
            Node fileNode;
            VersionManager versionManager = null;  
            
            if(folder.hasNode(filename)){
                fileNode=folder.getNode(filename);
                if(!overwriteIfExist) {
                    //create new Version if file exists and should not be overwritten
                    versionManager = session.getWorkspace().getVersionManager();  
                    versionManager.checkout(fileNode.getPath());  
                }
                try {
                    JcrTools jcrTools = new JcrTools(false);
                    jcrTools.removeAllChildren(fileNode);  
                } catch (Exception e) {
                    LOG.error("Error while removing children nodes of node "+fileNode.toString(),e);
                }
            }
            else {
                // Create an 'nt:file' node at the supplied path ...
                fileNode = folder.addNode(filename,NodeType.NT_FILE);
                fileNode.addMixin(NodeType.MIX_VERSIONABLE);
                versionManager = session.getWorkspace().getVersionManager();  
                versionManager.checkout(fileNode.getPath());  
            }
            
            // Upload the file to that node ...
            Node contentNode = fileNode.hasNode(Property.JCR_CONTENT)?fileNode.getNode(Property.JCR_CONTENT):fileNode.addNode(Property.JCR_CONTENT, NodeType.NT_RESOURCE);
            if(!isNodeExist(session, contentNode))
                contentNode=fileNode.addNode(Property.JCR_CONTENT,NodeType.NT_RESOURCE);
            Binary binary = session.getValueFactory().createBinary(fileContent);
            contentNode.setProperty(Property.JCR_DATA, binary);
            session.save();
            
            if(versionManager!=null) {
                versionManager.checkin(fileNode.getPath()); 
            }
            // Save the session (and auto-created the properties) ...
            return true;
        }
        else {
            LOG.warn("Could not find folder with given path "+path+" in the repository!");
            return false;
        }
    } catch (RepositoryException e) {
        LOG.error("Error handling the repository while adding file "+filename,e);
        return false;
    }
}

Do you have an idea why this exception is occuring? As can be seen in the code, I am executing session.save() after writing a new file or writing a new version of a file. Thank you for the support in advance.