Hi I also had the same problem up till about 10 minutes ago. I have been pulling my hair out trying to resolve the problem my self when in desperation I rang support.
It appears i was using the wrong settings for GPU acceleration. As I run a Mac and have an Nvida graphics card in it I was told to change to CUDA then delete all preview files.
I did this and am now rendering videos without the red flash.
Hope this helps.
Cliff
