I still cannot install my App.

For 2 days I have rebooted my phone and formated usb.

I have loaded the keystore from apptomarket into MIT appinvento 2.

I still get the error "App not installed" what am I doing wrong?

Since I use apptomarket nothing seems to work anymore.

Even the original app from MIT appinventor 2 does not install.

That is not AppToMarket issue. 

If you have uninstalled it, then it doesn't exist on your device anymore and it won't have anything to do with other apps not installing.

Try installing SAME MIT AI .apk onto another device and see if you get same issue.

Yes I have tried on a tablet Samsung Galaxy. Same issue.

I suppose I am doing something stupid, but what could it be?

I think it is an App Inventor issue. My Kaspersky deletes a file after decompiling thinking it is a virus. From what I read in forums this is not the case. Some people have the same problem with AVG.

I think it is an App Inventor issue. My Kaspersky deletes a file after decompiling thinking it is a virus. From what I read in forums this is not the case. Some people have the same problem with AVG.

in this case it's a virus scanner issue, because it detects a false positive

you can contact the Kaspersky support and notify them about this, so hopefully after they updated their virus signatures, this will not happen anymore...

You are right. After zwitching off Kaspersky its running on my Phone. On the tablet I still get "Application not installed".