You are working on preparing dashboards or uploading workbooks to your SharePoint site for usage with Excel Services. Every now and again, even though you are clearly interacting with the session, EWA will suddenly tell you that the session has timed out.
Under some circumstances, the server may decide that it needs to recycle itself or stop servicing requests targeted at a specific workbook. In those cases, the server has no way of distinguishing between a timed-out session, and invalid session (a session identified by an invalid string internally - the same thing used by Excel Web Services) or a session that died because of the reasons just cited.
Because of those reasons, the server gives the most generic session-related error it sees.
What to do:
Nine times out of ten, this occurs because a workbook is corrupt in some manner or contains something that Excel Services otherwise does not expect. Identifying the problematic workbook and then using the forums to send it (scrubbed of any private info of course) would be a great first step at trying to resolve the issue. Also, if the workbook has recently changed, it could be useful to go back to an earlier version and see what change actually caused the issue.