Translucent window rendering error

[Chief_Jeffrey]

I'm running a ray trace of stained glass windows illuminated with sunlight, and something strange is going on.

 

When viewed straight on, the windows appear normal. But when viewed from the side, they get dark in patches, as shown here: 

 

            

 

The stained glass is a translucent material with a picture of the pattern (it's too complex to model with individual objects).  Here are the settings, which are the same for all three panes:

 

 

The right window frame is also glowing very brightly, even though the three frames are identical copies of each other.

 

Any thoughts on why I'm seeing 1) partially black windows or 2) an extra bright wooden frame?

 

I'm using Premier X12 on Mac OS 10.15.7.

 

 

Thanks!