I can confirm there is an issue with sheilded docking ports. I've also encountered the problem when docking a part using a tug to a station and not being able to use the "undock" command (the game ignores the button being clicked). This even happened to me last night when docking a part that had it's own command pod (so that isn't the issue). The only way to fix it is undock the part again, then undock the tug from the part, then re-dock with the part and finally re-dock with the station and you should be able to get yourself free. This has only ever happened to me when a shielded docking port is involved.