If I modify a CD drawing and immediately want to save it (save from menu or command-s), then I get the error message: "That document is already open and modified perhaps by another application" and a CD_Temp.# file is generated. If I click "Stop" and then the closing button of the document window followed by "Don't Save", the document is closed. Reopening shows that savings occurred correctly.
Your anti-virus protection is causing the problem. Contact the vendor to resolve the problem. If you are using Norton, the following tech note from their web site applies: