Hi Jens,
now you catch me in the corner 
When you delete application from the IDE, the application is not completely removed from the account(only binary is undeployed but the configuration stays).
This is an issue that we know and work to fix it.
In order to completely remove an application you should delete it from cockpit.
Other option to remove your applications is to use CLI tool - undeploy command.
About cockpit - try to remove all cookies from hanatrial.ondemand.com and to connect again (this is just guessing, as I do not know what may cause this hanging).
Best,
Dobri