Same here. I do not want to create another user account to just access MY files. I am the Admin.
I had to grant permission for my account. My account shows up as letters and numbers and says "Account Unknown". Not cool. I'm not sure what happened during the update, but I'm able to save to all my folders after I did this fix. It said I had some error with the other folders that wasn't Documents, but I'm able to save to them after I kept pressing continue.
DO AT YOUR OWN RISK:
Right-click on Documents > Click on Security > Edit > Add...
Enter your login name. I.e. CamiKitti would be mine unless you have something like Sn00pDwg.
Click Check Names. It will underline if valid. Press OK.
Press continue until it's done.
After it's done, your user account should be in there.
It should look like this:
Mine: CamiKitti (CAMIPC\CamiKitti)
Same issue here, upgrading from 8.1; my account became listed as "Account Unknown". I just now added my account to Documents via the security properties dialog, and while now I can save to the root of Documents there were a some errors allowing access to many of the sub-folders. Hope I can find a fix for it.
My other PC just got upgraded to 10 from 8.1, and this user account problem did not occur.
Update: On the problem sub-folders, I had to add my account to those permissions manually. A bit of a hassle.