After converting my Mx444 app to Mx581, it won't start in the Mendix cloud test environment. The app runs perfectly in development. Starting the app there gives no errors or issues in the console. Deploying to the cloud slot also no problems. When I start the app it throws the error "Starting AppContainer failed, internal cloud error." Nothing is entered in the logfile. Starting the app in Eclipse also no problem. I have removed after start-up flows just in case, but to no avail. What could be the problem?
I had a quick look at it, and it seems that a customization (custom java permission settings) that was made to your application in the past conflicts with the Mendix 5 deployment process.
I suggest you open a support ticket for this issue. It's not an error in your project.