Error during request execution: Code: 400, Reason: invalid, Message: Invalid recurringEventId value.
This error means we could not migrate one or more instances of a recurring calendar event.
This is caused either by corruption in the recurring calendar event itself, or by invalid data returned by the source that, in turn, impacted our ability to create the item at the destination.
First, enable “Log Subjects of Failed Items”. How do I get the subject of failed items?
If the migration completed despite these errors, run a “Retry Errors” as a second attempt at migrating these items. If the migration failed, on the other hand, try starting the migration as you did before.
If the errors are still unresolved, log into the source and try to open the offending calendar items. If so, either save or re-create the calendar event in the source to see if that resolves the problem.
If the offending calendar item(s) still fail with this error, it means there is a potentially unsolvable issue with the calendar event and you may need to filter the calendar where those events are stored.
To learn more about filtering folders, please refer to the following KB article:
How do I filter a folder?
Mailbox, Calendar, Error lookup, Migration Help