If no disc is in a drive, and AnyDVD is enabled, AnyDVD will block communication to the drive. I don't know how Daemon Tools' "mount" utility communicates with the driver, but it looks like they do it in a way "blocked" by AnyDVD. (Virtual CloneDrive does it in a way, which is not blocked).
So, this is quite normal. You can simply disable AnyDVD (closing shouldn't be required), mount your image and enable it again.
If you don't need AnyDVD to "work" with Daemon Tools, you can exclude Daemon Tools' Drive in AnyDVD settings.
If you want AnyDVD to work with DVD images (jump to title menu, etc.) you can install Virtual CloneDrive instead.