Less disagreement or fewer disagreements?
Whether the word "disagreement" is countable or uncountable?
Which usage is proper?
This policy will cause "fewer disagreements", or, this policy will cause "less disagreement" ?

Question

Less disagreement or fewer disagreements?
Whether the word "disagreement" is countable or uncountable?  
Which usage is proper? 
This policy will cause "fewer disagreements", or, this policy will cause "less disagreement" ?

Answer

In your examples: "...policy will cause fewer disagreements or; this policy will cause less disagreement," both are correct. Other examples: "No disagreement," "No disagreements," "Much disagreement," "Everyone disagreed," "We were not able to resolve our disagreements," "They could not agree," "We agreed to disagree."

Answer

Yes, it can be countable or uncountable... so we can say either.