I had that same problem before. Use legacy connection was for sure a solution for me, but that was not the only thing I did. I looked in to the security setting of my browser for this website and give adapted permissions.
This was my topic
After a while, I notice that there was a problem in the project. I did a save as and deleted some bugs that were in it, and now I use the companion like I did before.
though this is old issue I am sharing my solution
I restarted pc thrice didn't work,
I restarted the phone once worked 
I can explain you at least what is the problem for me.
USB connection:
After the installation of the SW and drivers and setting the phone in debug mode, when I connect to the USB I can only see a white screen in my phone with a grey band in the top.
Legacy mode:
Stop loading at 10%
"10 found Companion" written below.
I don't understand why the companion will not connect.
Looks like the problem is back!! The workarounds (emulator, legacy connection) also are not working for me---so here's hoping the problem gets resolved soon. Just hoping to get confirmation here that the problem is on MITs end this time around as well.
Same here
This issue should be resolved.
