You forgot to pack texture before sending your .blend file.

It looks like sun higlight is blocky because of settings into Indirect Lighting panel.
If you increase Filter Quality and Cubemap Size settings to maximum, you should obtain a much better result.

@ronan ducluzeau (zeauro) added two links
Sadly filter quality just blur and pieces.
And even if blocks blurred and mixed, there is too high and sharp pinch to the pole of world. Check second video.

pinching pole is not depend on rotation of environment, or object`s rotation. Glitch appearance is not depends on streight of world illuminance

You have obviously wrong settings. Try to set the "clamp glossy" value to 2, increase the "Cubemap Size" to at least 1024 and set the "Filter Quality" to 8 and it should work!

@Tibor Horvath (tibor81) clamp is just cutting brightness of artifacts. But wrong highlight blur is still there. And resolution of map makes em even sharper
Here is Clamp to 1.0, roughness 0.27, cubemap 2048

Here a Pic with roughness set to 0.2. Looks fine to me! Don't Forget to set the Filter Quality to 8. It makes a big difference!

@Tibor Horvath (tibor81)
Filter 8, clamp 2 (because lower clamp looks reaaly dull), 512px cubemap
Pinched pole still there.

AFAIK it is impossible to bake different cubemaps separately with different settings to match every material with different roughness.
And anyway it is not a good workflow.

And it is still bug or wron algorythm, that make filtering distribution weird.