There might me multiple reasons for low quality of shadows in certain places, but looking at your current settings I’d start with the following:
- Decrease Flood dark limit to the default value = 0.02 (see: Light Blotches in scene - #2 by wojtek for more information).
- Use Isolated shadows option on the image captions. The lightmap resolution of the wall might be too low to represent such detailed shadows as from the captions.