As Ronald wrote: "And double check that you have a home page for the administrator role."
Further reading about user role based pages can be found here: https://docs.mendix.com/howto6/setting-up-the-navigation-structure
An easy check is to have a look at your navigation which can look like this
in the column ‘User Roles’ to the right, you also see which user roles have access.
Did you check your settings in Project security?
And double check that you have a home page for the administrator role.
This is what my navigation looks like