The simplest solution is often to restart ConQuest: this can happen because ConQuest has lost contact with the file share.
If that doesn't work, perform a Check Job from the Tools menu on both projects, the one you are copying from and also the one you're copying to.
In some cases, however, the error is given because the system is timing out trying to perform the procedure.
Click Options and check the Project Total settings at the bottom.
Make sure this is not set to Update on Every Change as this requires the system to do a lot of unnecessary work which could be getting in the way.
Click OK. ConQuest will restart. Try the copy again.
Error 104 messages can also be caused because the Reference option is not selected when detailing what information is to be copied over to the new project.