Depending on the version of Dynamics GP and environment they are using there are a few potential fixes for the most commonly experienced errors:
1. Close Dynamics GP, and use the Task Manager to ensure that there is not an additional instance of Dynamics.exe still running that didn't get closed properly.
2. If you're on a terminal server, this error will sometimes occur if you're running Integration Manager and Dynamics GP as published apps.
3. In some versions of GP this error can occur if you're using the default option for Windows Command Display. The default setting for the buttons at the top of a window is "Action Pane". You can change this setting my navigating to User Preferences under the main Dynamics GP Menu. There is a dropdown on this window for Windows Command Display, and if you change it to "Menu Bar" it can cause this error to stop appearing. You have to completely close GP for these changes to take effect.