The reason is the server and mobile device now have different passwords. Before setting up a mobile device you must be able to successfully login on the PC, if you cannot then reset you password and login before setting up the device.
The reason for this is if you setup your mobile device with password 'ABC' and reset your password on the server to 'XYZ' then the server cannot authorise access to your account as the two passwords will not match.
What do I do? Simply uninstall the mobile software and reinstall it.
Make sure you delete the old serial number and raise a new one in the 'Your Business' -> 'PDA Serial Numbers' form.