Tumblr Tags Not Working – How to Fix Old Tags on Tumblr

I have a comic book Tumblr where I track every comic book I read (sidenote: I know this is useless, it's Tumblr). I keep track of the comics with tags (series title, publication year, writer, artist and publisher). I've noticed that some of the oldest tags on my Tumblr have stopped working even though they still appear on the post. For example, when I click on "X-Force"…

…it tells me the URL I requested could not be found.

I guess there's a limit to the amount of tags Tumblr can remember? Is there anyway to free up tag space (like deleting typo tags)? Is there any way to get these tags to work again?

UPDATE: In trying to find a pattern between which tags do and don't work. Here are the first tags I ever used (back on 1-1-10):

• Uncanny X-Men (used approx. 55 times; does NOT work)

• 2010 (used 477 times, still works)

• Matt Fraction (used 72 times, still works)

• Terry Dodson (used 14 times, still works)

So it doesn't seem to have to do with how many entries are in the tag. I think it might have to do with the hyphen in the titles of some comics (Uncanny X-Men and X-Force do not work). Is that it? These used to work, I swear (I would not have continued to use tags with hyphens if they did not work).