I had the same thing, and it was a result of permissions not being set properly for the dashboard.
On the Settings tab for your Dashboard page, the role (or roles) for your user should appear under "Who can access this page?".
If it doesn't, I'm not totally sure which of the following steps fixed it for me, but something worked!
First, make sure the "dashboard" permission is selected for the role (the permission should have been created by the module). And of course, check the role is assigned to the relevant users. Then go to the Modules page and click the Refresh button.
Then if that doesn't work, you may need to go to the permissions for the Admin template and under Access change "manage view and edit access" to Yes. Then hit Refresh on the modules page again. By the way, a quick look inside the Dashboard module indicates it uses the technique Ryan suggests at https://processwire.com/talk/topic/276-creating-new-admin-page/, which may help with debugging.