Symptoms

On E-mail Event formatting a string Datafield is generating error on recently upgraded environments but doesn't generate error on Dev environment. This error was actually generated on 4.6.6 env. "Input string was not in a correct format."
 

Diagnoses

Generally this type of issue should appears if there are Regional setting of the K2 service account not correct.
 

Resolution

You can start to verify the culture configuration of K2 Service Account: it should be EN-US and with decimal separator the point '.'. To check this, please connect with the K2 service account on windows and check the regional setting of windows.