SOLVED.
The problem was that my graphics drivers were out of date! I updated them, and the ghost of no longer haunts my game development effort!
I’m getting weird lens flair effects all over the default example projects.
Here is what it looks like (C++ puzzle project without any modifications): – but it’s the same in other default project templates as well.
I’ve built from source code on Linux, repo branch 4.10. My NVidia driver version is 340.96 x86_64. The about dialog lists the version as “4.10.4-0+++depot+UE4-Relesaes+4.10”