RDS (formerly known as RDP) 7 released a new Security feature called Network Level Authentication, or NLA. This option needs to be supported by both, the client and server components for a successful connection to occur. If your remote computer has the NLA feature Enabled, and your remote devices do not, you will need to disable this feature.
Right click on "My Computer" and select "Properties".
Click on "Remote settings".
Click on the "Remote" tab.
Under "Remote Desktop" select the radio button next to "Allow connections from computers running any version of Remote Desktop (less secure)".
Click "OK" to Save.
Also check to make sure you don't have any sort of VNC Server installed (like TightVNC, Ultra VNC, RealVNC, etc). If you do then you should uninstall it to ensure that RDP connections work properly.
This can also be caused from the client side if NLA is enabled on the server side, but not on the client side.
On your phone or tablet, go to your Connection Settings and Enable NLA. Also enter your username, password and domain if necessary, otherwise you will be prompted with the "Please enter your NLA credentials" screen..