Lighting Flash bug
Note: This does not only happen to Elementalists. I observed that a few or perhaps every teleport skills can trigger the inability to move afterwards. I experienced it on Mesmer, too, and heard it from thieves as well.
It’s a pretty rare issue and it’s very hard to reproduce it. A weapon switch seems to fix it though, and so does a “forced” move (by getting feared or by using elementalist staff fire skill 4).
Abaddon’s Mouth (DE)