If the Crystal Report uses an export:
  1. Open the report in the Crystal Report application
  2. Refresh the report to produce the error (this should also open the formula causing the error)
  3. Add a check for division by zero
Example
Original formula code: 
x / y

New formula code:
if y <> 0 then x / y


If the Crystal Report connects directly to the database tables:
  1. Open the stored procedure in SQL Server Management Studio
  2. Debug the code to see where the error is being produced
  3. Include a check for division by zero