Why do I get Error on PrimeOuput() in SQL Integration Services?

I am workign on a "sort-of" Datawarehouse for a customer, and I am importing all data from some AS400 tables into SQL Server Tables, using an ODBC connection.

So far I have created about 10 packages that mostly do the same: they all connect to 2 AS400 tables, they make a query and their result are appended in a single SQL Server Table.

Everything is working EXCEPT for one of these packages, which looks almost identical to the others (same logic, of course different tables involved).

I always get the same error:

[SSIS.Pipeline] Errore: SSIS Error Code DTS_E_PRIMEOUTPUTFAILED.  
The PrimeOutput method on Origine ODBC 1 returned error code 0x80004005.  
The component returned a failure code when the pipeline engine called PrimeOutput(). 
The meaning of the failure code is defined by the component, but the error is fatal and the pipeline stopped executing.
There may be error messages posted before this with more information about the failure.

This is all I get, no additional info. In the connection manager for both AS400 sources I can succesfully get a preview of the data. But when I excute the package, I always get the same error. Any idea what this error means and how to solve it?