Raspberry 4 and OpenGL

I have a Raspberry 4 with OpenGL enabled, updated all drivers etc. Checked everything I could read about the subject. ‘glxgears’ (an OpenGL demo) runs smoothly with a performance of about 60 FPS, exactly as the monitor refresh rate.
But when I want to run the simplest test in Xojo - a simple scaleable triangle on an OpenGLsurface see Tutorial 1 - this Surface is black, and stays black whatever I try.
Needless to say, running exactly the same example on my iMac or Windows 10 computer, everything is as expected.
Any ideas anybody?

Thanks, Joop

Probably some bug. I see an unsolved case since 2018 for Linux and OpenGl here feedback://showreport?report_id=53118