RE: Internal error occured while processing the mixdown...
What effects did you use? Were any of them "freebies" or demos. If so, delete them and see if that helps.
Computer: Core2Quad, 8GB/3.5TB+, Windows 8.1 Pro 64-bit, Sonar X3 Producer, Delta 44, Cakewalk UM-3G
Instruments: SL-880 Keyboard controller, Korg 05R/W, Korg N1R
Axes: Fender Stratocaster, Line6 Variax 300, Ovation Acoustic, Takamine Nylon Acoustic, Behringer GX212 amp.
Outboard: Mackie 1402-VLZ mixer, Digitech Vocalist WS EX, PODXTLive, etc...
Controllers: Korg nanoKONTROL, Wacom Bamboo Touchpad