Inspect the first button and in the Button (script) component find View Field State Values. Expand this and expand the second ImageComponent.color. The value of this property should be "0,4901961,0,4901961,0,4901961,1".
When running the game an exception is thrown on startup and when triggering mouseout on the button.
This bug caused the decimal separator to be comma instead of a dot.
Resulted serialization was for example "0,4901961,0,4901961,0,4901961,1". With this fix it is the correct version "0.4901961,0.4901961,0.4901961,1".
How to reproduce: