I managed to reproduce these “Lookup failed for EventModel” warnings when opening a test project. I traced the cause, and the good news is that they are harmless. They won’t affect anything in your game.
If you’re interested in the details…
When the user opens an Events folder in the Content Browser, the plugin normally receives a message from the UE4 editor to refresh the asset tags. You can see these when you hover over an event: Oneshot, Streaming, and 3D. Internally, the plugin calls the Studio API to obtain the info about each event. This all works fine. However, something changed in Unreal 4.10 and now the plugin also gets an asset tags refresh when the project is opened. As the plugin has not yet loaded the banks at this point, the call to the Studio API returns an event-not-found error and logs the “lookup failed” warning. The plugin handles this error gracefully, so there is no harm done. Later the asset tags will get refreshed again when the user opens an Events folder, by which time the banks will have been loaded and the correct info will be retrieved.
We will make the necessary changes to the UE4 plugin to eliminate the log spam in the next release. In the mean time, you can safely ignore these warnings when opening a project.