We've migrated a MOSS 2007 Site Collection into SP2010 as a Site Collection under a tenant in a multi-tenancy set up. This migrated SC gives an error on checkout through the UI that states that the user either doesn't have permission to check out the document or it is already checked out to another user (neither is true).
Checkout fails using the item's drop-down menu or the ribbon. Checkout also fails on this SC regardless of if the content was migrated or if it's new, and checkout fails in newly created Doc libs as well.
We can successfully check out multiple documents at once through the UI, and individual documents through the client application (Word, Excel, etc.). We can also successfully check out documents at the tenant level and at a newly created sibling Site Collection (no migrated material). We can check in items that have been checked out. Finally, if we upgrade the SharePoint OpenDocument class add-on for IE to version 14, it works (However, I want to reiterate that having the version 12 dll doesn't have an impact on any other SC but this one).
Mass-upgrading the dll isn't really an option for us, and since checkout works in other SCs we think this may be a configuration issue dressed in a client issue's clothing. This forum thread shows we're not alone in this issue, and we've tried most of the solutions listed here (we're getting fiddler traces now). Anyone else have any ideas?