Try going to this folder: C:\Program Files\Steinberg\Cubase SX\MIDI Port Enabler, right-click the ignoreportfilter file and click Copy.

Go back to the C:\Program Files\Steinberg\CubaseSX and right-click on a blank area and click paste. This will copy the file into this directory.

Also, you might try going into Devices - Device Setup - All MIDI Inputs, and disable all emulated ports by clicking on Active Yes and it will change to No.

Make sure you have the most current MIDI interface drivers and the latest Cubase SX version.