This page contains some technical issues often encountered when installing OpenMM on Windows, and how to solve them.
Windows users may not be able to load trajectories with NGLView. This is due a bug, for which the fix can be found here: nglviewer/nglview#863
This may happen on Windows when trying to import a module in Python whose DLL file is opened by another process (and not properly closed).
The simple solution is to restart the kernel of your notebook: select the Kernel menu and then Restart.
If that did not help, try rebooting your computer.
This may happen on Windows when the file was opened previously but not closed. Restarting the kernel of your notebook should fix this.