Hi,
Finally it got solved for me.
I unrooted the phone with rootgenius (I am not sure if that was required).
What I think solved it, is that I had to install the android app Mobileuncle MTK Tools (from google play).
In that app I clicked on reboot to recovery
I had the file update.app on my...
I am sorry for writing in English.
I have the ame issue, I foud out that when I press the Volume- button while it starts AND it is connected via USB to the PC, it goes into the boot menu but then I need fastboot and I have to enter in the cmd promt: fastboot continue.
Then the phone boots, but...