Hm.
1. Confirm that you have installed the latest version of nUI (5.07.26).
2. Start WoW.
3. Before logging in to your character, click the "addons" button.
4. Click "disable all addons"
5. Enable nUI (only).
6. Log in to your character. Does the problem persist? If not, you have an addon conflict. If so...
7. Exit WoW.
8. delete or rename /world of warcraft/wtf/<account name>/<server name>/<character name>/SavedVariables/nui.lua and nui.lua.bak to something else (while WoW is not running).
9. repeat steps 2 thru 6. If the problem still persists, report back here.