If you go to the Environments page of your app in the developer portal, there is a blue button with 'View Live Log’. This shows the log of the app running in the cloud. Have this logging page open and try to run in the cloud again.
P.S. Sometimes it helps to hit 'Clean deployment directory’ in the modeler under Project in the menu.
I have encountered this several times. One of the times was because of an error in the after startup microflow (found in the project settings, under the runtime tab) which is similar to what Mike Kumpf has described below; however, when I face this error it is mostly an issue with the JAR files, so you can try to create a Deployment Package locally from your Mendix Studio Pro and see if it’s an issue with creating the deployment package instead.
Did the “Check latest build output” on the environment page show you anything?
Additionally you can export a package of your project and upload it as a new team server project. If for some reason the deployment doesn’t fail for the new team server project, then its safe to assume its an issue with the sandbox and support should be able to help you out.
Another good thing to try is to debug your after start up microflow (like everyone else was mentioning). I usually put it as a button on a dev page when running locally so I can debug it.
This time the solution was to set the app’s security setting to production and extend some userrights.