Cannot launch LR 6 standalone after upgrade

hi, I have downloaded LR6 standalone (previous version 5.7.1) on Windows 8.1

installation and registration is done without any problem or error message.

when first launch launching LR6, I get asked to upgrade the catalogue, this is done again without error and I get the welcome message for using LR6 for the first time.

Thereafter LR6 freezes, every time. In the background I see the last image I was working on in the previous version.

I have re-installed LR6, removed all .lock files but everytime the same thing happens.

By now I have multiple catalogue in the folder - see screenshot below.

Please help!

Thx

Thomas