New user accounts can't be opened: profile problem

My PC came with W8.1 installed, and I have always used just the one account that appeared on setting up, with admin rights. Now I am taking notice of advice to run the account at "standard" level instead, because of the extra protection from any external attempts to infiltrate the PC. But the rule is (of course) that there has to be an admin account. So I want to create a second account, give it admin rights, and then demote my existing account to standard.

BUT the problem is that if I create a second account (whether admin or standard), I find it can't be opened: on logging in, I always get the message "User profile failed the sign-in, user profile cannot be loaded". The same happens if I activate the existing Guest account and try to open it.

A possible reason for this seems to a corrupted Default profile (because new ones are based on this) -- but can it really have been corrupted from new?? And if so, where can I obtain a replacement profile (not having another W8.1 machine)? Might replacing some of the Default profile files with ones from my working account do the trick?

Are there other possible causes? And possible solutions that won't make me nervous about losing access to everything?

I also notice that although, first of all, I modified the name of my working account, the name listed in File Explorer under C/Users has not changed -- should it have done?

Thanks for any help with this.

Just to add that in the C/Users/Default folder, I have tried replacing the set of three NTUSER.DAT{.... files with (a) the identically named set I found in my own C/Users folder, and (b) with the differently named set that was also there. Each time I created a new account, tried to sign in to it, but still faced the profile problem as above...