Unless it is accepted to both the parties, an (1)/ arbitrator would be of no (2)/ use to settle this dispute. (3)/ No error (4)

Answer

1

Explanation

acceptable will replace accepted/by will replace to because -
accepted (Verb): to agree/approve of something acceptable (Adj.) : agreed/approved by most people in a society, agreeable, capable of being accepted Look at the examples given below:
The princess accepted the offer. (Active Voice)
The offer was accepted by the princess. (Passive Voice)
The offer was acceptable to the princess. Hence, acceptable to/accepted by will be used. Hence, Unless it is accepted by/ acceptable to both the parties, an is the right usage