I eventually found the plist file and deleted it, but it did not resolve the problem. Then I deleted the entire connection and recreated - still didn't work.
I'm able to enter the same site and access the same document through the safari browser with no problem, so it doesn't seem to be a permissions issue on the server side. I'm also able to connect and browse sharepoint files through Document Connection. It's only when I try to access the document through Document Connection that I run into the problem.
The version of Document Connection is 14.3.8. The Office 365 desktop suite with Document Connection was just downloaded two days ago, so it's got to be the most recent version.
Any other help appreciated.
Thanks,
John