Windows 10: Windows 10 start button critical error

I notice that the start button problem continues - ie when I press the left mouse button over the "start" button at the bottom lhs of the screen, I get the same Critical Error message telling me to sign out (I have to, because I can't get any further), and sign in again, and that "we'll try to fix it next time". "we" don't, and I have no access to much of the toolbar at any time.
Having used Windows 8.1, which was pre-installed on my HP 23Envy desktop, I didn't have any real problems, so my question is 2-part;
1. Has this really annoying and major Windows 10 Critical Error been fixed yet?

2. Is there a way I can re-install my Windows 8.1 without losing all my stuff?

I've tried - and failed - with every solution offered, to fix this error including the (deliberately?) confusing so-called MS Help services, and end up being diverted into signing up to a MS account, X-Box stuff etc., etc.

I have always doubted Microsofts commitment to its customer base. Their "support" amounts to FAQ's which do not address the issue I started out with. If there was another operating system other than Apple, I would have dropped MS years ago. If Linux was mature enough, I'd have a go with that.