Hmm... I have similar entries myself. Although perhaps my entries from LJ didn't have more than one tag, since I only started getting into tags in a big way after the DW migration. So, I wonder if perhaps this particular tags data was brought over as an array...

The other possibility is that somehow the tag is being treated as binary data because of some unicode snafu, and the field wants either null or a string...
I'm going to run a brief experiment and get back to you.