When running 1.7.x if we tried to open a link in to the admin section but we weren't logged in, we could login and it would automatically bring us to the page we had first tried to access.
After upgrading to 184.108.40.206, after login we are always brought to the default page (in our case that's the Orders page, but for most people it is probably the Dashboard).
For example, clicking a link to edit a product: http://example.com/index.php/customadmin/catalog_product/edit/id/481/
If we were not logged in in 1.7.x it would ask us to login, then bring us to that that. Now it brings us to the default Admin start page.
Update 3-22-2016 - More Details
Cache and compilation are disabled.
I have tried disabling all modules that have
<use>admin</use> in their config.xml files. In fact I have tried disabling all non-core modules.
After doing so I log out, log back in, copy a URL to an order, log out, visit that URL, login, and I still get rerouted to the Dashboard/Orders page.
I have tested this on a new install, and a fresh install (new store) - both have the same result.