Windows Phone 8.1: Flat example does not start due to shader errors

Starting the flat (Qt Quick) example fails with shader errors:

<code>
QEGLPlatformContext: eglMakeCurrent failed: 3001
QOpenGLShader: could not create shader
QOpenGLShaderProgram: could not create shader program
QOpenGLShader: could not create shader
QOpenGLShaderProgram::uniformLocation( imageTexture ): shader program is not linked
QOpenGLShaderProgram: could not create shader program
QOpenGLShader: could not create shader
QQuickCustomMaterialShader: Shader compilation failed:
""
</code>

The problem did not exist with the Qt 5.5 release.