The 502 error message can be caused by different factors in Salesforce. If you encounter this error, you are welcome to contact our support team along with the exact date/time that the error was generated, so that we can research further. Without knowing when it happened exactly it is hard to track it down.
This error occurs occasionally and is not always indicative of a systemic issue.
The good news is that a 502 error should be recovered with the next Salesforce sync and any items that would have been included at that time should then be picked up and synced over.