Are you using the visual text editor? If so, switch to teh code and see if it comes up.

Or a client to post? If so, try posting through the online LJ interface.

Or maybe try logging in with a different browser?

You can also try posting it, then editing - the poll will end up being one line pointing elsewhere, but you ought to be able to type stuff on top of it in that case.

If none of that works, I think LJ support is the next step, because that's odd behavior.