To be clear the DC will run on 64bit systems. We don't support for 64bit specifically because DC was not fully tested on 64bit systems, and because some licenced moduals are not certified for 64bit configurations. A matter of timing in development. This can lead to non-resolvable issues. Future versions of DC and LTE will support 64bit configs.
I don't have a specific resolution or cause for your circumstances. To delve deeper, Are you using dual or single monitors. And if dual, are you dragging from one monitor to the other?