You will be prompted with 'Invalid Time Stamp' error when the difference between the UTC timestamp generated by your server and ours is more than 30 seconds. Ensuring that your servers stay in sync with the NTP server (https://en.wikipedia.org/wiki/Network_Time_Protocol) will sort out this issue.
If you continue to face issues, kindly write to support@freshdesk.com and one of our agents will assist you further.