Another possibility is to try to detect what else you're running may be using the USB port of the power meter. The error displayed indicates something suspicious is happening on the system with USB ports.
There are 3rd party programs out there (for example, backup programs for USB hard drives) that may run at all times and query all available USB ports for devices. To check if this is true, use the Process Explorer, and search all processes for "USB" to check if any process is using your device. (While you're in the state when it does not work, of course).
If there are some such programs, try terminating them from the process explorer, and run the SFP again. Assuming that there are such programs on your machine, evaluate whether or not you need them running at all times, and try configuring them differently so that they do not interfere with the communication.
-Serge
