On my system the following directory seems to contain all the model COA definitions in xml format: C:\Program Files\Sage\Accounts\Configs\GBR.
If you open then in a text editor you can see what is in there and maybe replace the corrupted one with a standard one from a different version. Maybe You could replace your OwnCOA.XML with a standard one from another system.
I'd be happy to send you the above set if it would help you rebuild a standard set to get going.