A little research found me this post by Tim Wappat where he explains this is an error due to a network connection glitch.
Network interruptions are known to cause issues with Microsoft Dynamics GP; this is because it takes a persistent connection with SQL Server. Not all network interruptions are due to a problem; some of them are because clients lock their PC when they go to lunch, but left Dynamics GP running. When the PC has been locked for a few minutes it goes to sleep. Whe the user wakes it up and resumes working, Dynamics GP makes a new connection to the database which is assigned a new SID, but Dynamics GP still thinks it has the original SID.
After advising the client to have users always log out when they are leaving their PC for a while, they have reported that users no longer see the error.