I've literally just updated from r16 to r19 and I wonder if someone could offer me some assistance.
I've ported a project I've been working on across to R19 and I've found a strange viewport problem with the preview reflections.
I've uploaded the viewport view and a render of the same frame below:
It doesn't actually seem to be causing any problems in regards to the render but it's rather annoying having a strange purple splodge covering any object with a reflectance in its texture.
I don't think the preview had reflections back in R16, and I don't remember when the reflection tab changed in the materials editor, but the old material might not jive with the new preview engine.
You can turn reflections off in the preview - its in the Options/configure/EnhancedOpenGl tab.
It may also be the way your gfx card is working with R19 vs the way it worked with R16. You can turn Open Gl off in the C4D preferences to check.