MMOItems blocks are not compatible with ItemsAdder and vice versa.
You have to open the config.yml file of ItemsAdder and disable REAL blocks (mushroom).
config.yml
blocks: # .... custom: # .... mushroom: false
After applying this change you won't be able to create ItemsAdder blocks with type: REAL.
Other ItemsAdder custom block types will still work (for example REAL_NOTE).