We are in the process of upgrading new changes to our Salesforce org.
For this we are mapping some old record types to the new ones and are using data loader for the same.
When we run a report on activities we see that no user has the old record types - but i can see few participant records that still have the old records. The data you were trying to access could not be found. It may be due to another user deleting the data or a system error. If you know the data is not deleted but cannot access it, please look at our support page.