Your issue may be related to the drive you are attempting to start tensorboard from and the drive your logdir is on. Tensorboard uses a colon to separate the optional run name and the path in the logdir flag, so your path is being interpreted as \path\to\output\folder with name C.
This can be worked around by either starting tensorboard from the same drive as your log directory or by providing an explicit run name, e.g. logdir=mylogs:C:\path\to\output\folder
See here for reference to the issue.