To resolve this issue, us the following INI parameter:
CalMaxRecurrenceFix=<#>
The parameter value is a keyword that must be one of these single digits:
0 (default): The item is migrated as-is. (Migration to Exchange 2010 will fail for series with > 999 occurrences.)
1: Wizard converts the item to a recurring series with no end date, which migrates cleanly.
2: Wizard converts the item to a recurring series with 999 occurrences (the Exchange maximum).
Remigrate the data.