I also have problems on a imac 24" (10.5.4, 4GB ram, FCP6.0.4, QT7.5)
RedRushes and the R3D automator action both render white frames when rendering to Prores. Using Redline directly in the terminal works as intended as does RedAlert. Very weird...
Both my MacBook and MacPro works as intended.
Anders, can you send me the RSX and the equivalent command line that works for you?