When you view a lead’s activity history, you noticed a failed sync activity type with the following example:
This message is very general and can be caused by a number of sync related errors. In order to get more information, you will need to double-click the error message and a separate pop-up window will appear. Search for the section labeled: “SFDC Error:” like the following example:
Depending what the error message is, Salesforce provides a website that lists all Core Data Types used in API calls. You can access it by clicking here.