The first time I tried to install Windows 10, I had to revert to 7 because the Start Menu started producing Critical Errors.
Naturally, I searched the web for solutions. None worked, including the suggestions made by Microsoft about using a particular sequence of logins, checking system files for corruption, and creating a new user and transferring all my data to the new user account.
Eventually, I came across this link to a post of possible solutions containing the usual stuff, but someone also mentioned that their system started working correctly once they removed Dropbox. I found more corroboration in a few other places scattered around the web. As unlikely as it seems, it actually worked for me.
I find it incredible that the folks at both Microsoft and Dropbox seem unaware of this and haven’t fixed it.