Mark,
I reviewed your issue and when i tried it on my system, I received the exact same error "Owner is not valid", when I knew for sure the name in the CSV file was the same as the one in Centric CRM.
I reviewed the issue and its code and see that there is bug in the code which tries to map the owner name to a particular user in Centric CRM. I have fixed this issue and will submit it back into our source reporsitory and will be available in a subsequent release. Thanks for bringing this issue to our attention.
Back to what you are trying to achieve -- You are trying to export account contact records and then trying to import this back into Centric, expecting it to update your existing records. This will not be possible since the Importer available in the Accounts and the Contacts module is a Contact Importer. There are a set of predetermined Contact fields (available in the drop-down while mapping fields during import) against which you can map the fields in a CSV file. The "Contract End Date" is a property of the Account object and not the Account's Contact object. So this field cannot be mapped during the import and what you are trying to achieve will not be possible.
The only way that could be available to you at this point, to achieve updating the Contract End Date, is by using Centric's HTTP XML API described at:
http://www.centriccrm.com/ProjectManagement.do?command=ProjectCenter§ion=Wiki&pid=58&subject=Using+the+HTTP-XML+API
When you do a regular export to get a CSV file with the Account ID field selected, it spits out the ID that centric uses at the database level to refer to a particular account record. You can use the id and send an xml packet to your Centric server with the End Date info. Centric will process the packet and update the account.
If you are interested I could send you a sample packet which can be run against your server to update an account. This might not be straight forward but works for now.
- Ananth