I think this might be a Spyder/Qt issue.
Could you check, which pyqt version Spyder is using?
In pyqt5.7 some issues were reported of blank screens.
Sometimes you can resolve it by unplugging your external monitor, if you are working on a laptop, but it’s not a really useful workaround in my opinion.
Could you try to downgrade pyqt to 5.6 and try Spyder again?