We are having this same issue and I am finding it directly related to the /moodle/my:manageblocks setting for authenticated users. When I uncheck this setting to prevent authenticated users from Customizing the Dashboard it will bring up a blank page for users who have never logged in before.
If I set the /moodle/my:manageblocks to allow customizing the Dashboard, first time users will get the normal default dashboard.
Does anyone have a solution that will prevent customizing the Dashboard and allow first time users to get the initial default dashboard loaded?
DW