Have you tested to import another application, just to make sure the issue is not application specific?
Please also have a look at the help page for importing applications:
If the problem still excist I would recommend you bring this issue to our Technical Support for futher help and troubleshooting.
Thanks for your answers! The problem was related to a very interesting bug:
After the installation of qlik sense the root admin user
- was not able to open the operation and license monitor
- was not able to open and edit apps (even that one he created himself)
- was not able to import apps (as described)
I had to deinstall qliksense, delete the whole qlik sense directory manually (C:\ProgramData\Qlik\Sense) and to delete the created certificates (see this thread Qlik Sense QMC and Hub do not open )
Best regards, Constantin