You can get this error when uploading/organizing media.

If you have load balanced the web fronts, make sure that they have sticky connections active, otherwise session data is not kept between requests.

We recommend using Source IP as sticky method.

Another cause of the problem is that the session is timed out. 

The session is timed out either when the timeout period has passed (normally 20 min) (ie. the user has been inactive for this period of time).

The session is also timed out whenever the application pool is recycled since the session mode by default is set to in process.

To solve this, you can increase the timeout to a value that suits your needs

See example below:

From ImageVault 4.7, you can also change the SessionMode to something else than InProcess so it will survive application pool recycles.