You encounter error "Picklist validation error in field "Index Field". (Error code: 501)" 

it is indication that the value that you have for this particular index field no longer exists within that picklist.
Most likely this is due to an administrator deleting this value from the picklist.
To overcome this issue you can do either of the following,

  • Delete the value on the index field
  • Change it to a valid value from the picklist.