Today I succeeded in using qemu for repairing the qcow2-file just as was done in the case of Worpress.
Qemu discovered an enormous amount of errors, and after that the repair function of QEMU reported that it had fixed all the errors. And, when I tried the check of QEMU a second time it confirmed that no errors could be found. But the Joomla.app still refuses to start the vm.
So, my problem is still remaining. Regards, George