Just had a talk with @Akhil_Kulkarni on an Office Hour. I could not debug it myself, so asked Akhil to follow up here.
They are using a set of dependent dropdown custom field. Ticket update fails only when a particular field “Location A” or “Location B” is updated. The error message is generic, and difficult to debug. Code to make requests looks normal.
As discussed in the call, please have it checked on priority as this seems to be a very small issue and we need to deliver the APP to the customer by Monday.
@Akhil_Kulkarni,
AFAIK, you need to update all the dependent fields on the API call, otherwise, it will throw an error from the API side due to the validation failure.