Ah, now we have some more info. All video files are associated with a sidecar file in MC. That is a file that stores all the tags. When you write the tags from the library, it is this file that is written to.
Here's a plan of action:
Choose a file from your video library.
Do a "Update tags from library". If everything you say is correct, it should fail.
Now right-click on the file's row in MC's standard view. Go to locate> on disk external.
This will open the folder of the video file in Windows Explorer. You should see a "*_sidecar.xml" file for every video file you have in that folder.
Choose the one for the video you picked in MC, and remove it form that folder. You may delete it, or move it.
Now return to MC and go to the video file's entry in your MC library. It should still be selected.
Try running "Update tags from library" again. Hopefully it won't fail.
If you return to Windows Explorer, you'll see that MC has created a new "*_sidecar.xml" file in the place of the one you removed.
In the worst case scenario, you can do a search in Windows for all your "*_sidecar.xml" files and delete them, then rewrite them in MC.
If the "Update tags from library" fails again, you definitely have a write permissions problem, and hopefully we can help you with that.