Well, you CAN open your profile XML (or use the Copy feature in ISBoxer and paste it somewhere you can do this) and do a Find (Ctrl+F) for Ctrl+Alt+U in the XML

But you should have a few simple ways of narrowing it down. First of all, you should be able to go into the in-game Key Bindings screen in WoW, and try assigning any blank key binding (any empty box will do) to Ctrl+Alt+U. If WoW had something already assigned to Ctrl+Alt+U, like a WoW Macro assigned by ISBoxer, then it will tell you what it was at the bottom of the Key Bindings window! You can then un-bind what you just set to Ctrl+Alt+U.

The above should point you to what you need, however if you do this and it doesn't say something was previously assigned to Ctrl+Alt+U, then you've narrowed it down to just sending Ctrl+Alt+U without having assigned a WoW Macro (or another Key Binding). If you use the WoW Macro Library (under Game Helpers) for your WoW Macros, that would likely mean this particular Character does not have the intended WoW Macro Set assigned. To fix that, you would highlight the Character or Character Set for the WoW Macro Set (when you select the WoW Macro Set itself in the bottom left pane).