I got a package created in SSIS 2005 (32 or 64 bit, I don't know). When I imported the package into 2008, I got several warnings, which I ignored. I added some of my code to this package and then executed it. I now get the above error.
You might check whether it's an OLEDB connection. In SSIS 2005, the native SQL Client for OLEDB connections is at 1. In SSIS 2008 it's SQL Client 10. When I've done similar upgrades, there were some connections that remained at 1 even after the automatic upgrade. They need to be changed to 10.