How can i fix that issue?
This looks strange indeed.
What sun strength does this scene use?
Could you upload the scene, so we could take closer look at it?
I didn’t notice it the first time, but the Flood dark limit is set way too high for this scene. It’s 0.02 by default, and for most scenes it shouldn’t exceed 0.1.
Could you lower it to 0.05, run Post-process (re-bake is not needed after changing this setting) and check if all the dark lightmap pixels visible in the scene have been flooded?
You can find more information about this setting in this thread: Light Blotches in scene - #2 by wojtek