Chrome Portable as default browser: class not registered

Hi,

on my windows 10 PC (the problem existed also during windows 8.1 before the upgrade to 10) I am able to make Chrome portable as my default browser. Nevertheless, if I want to do a cortana search a window pops up which is titled as "openwith.exe" and says: "class not registered".

On my surface pro it works very well. I assume my registry is somwhow messed up but I have no clue which entry I have to fix. Do you have any tipps?