From @Sebastien Blaineau-Ortega on the mailing list:
"About the view modes, the reason for these issues we've always had is that we need to create an arnold session in order to load the .ass file and display it in the viewport. But we currently can't have mutliple arnold sessions in parallel. So if you have a render in progress (IPR, swatch render, material viewer....), then loading the ass file for viewport will fail. This is why it seems like it's failing randomly.
In the future, we should get rid of that multiple-sessions limitation, so standins viewport should become much more reliable."
Could this be the case?
