Sorry to resurrect.
I found this thread when Googling for the same issue (pixelated after OnResize(...), same D3D9 errors), and I found my issue was I was not calling OnResize(...) from the main thread. I was calling it from a background thread. After moving my call to the main thread, everything worked as expected.
I did not have to add extra calls to endScene()...beginScene() or reload textures as described by @robmar. Perhaps that was fixed since 2012. I am using a very recent 1.9 build from SVN.
Cheers,
Doug