Why did all my files suddenly become uneditable

After installing Windows 10, I am unable to edit any files.

"Document file such-and-such is locked for editing by:

Unknown User

Open document read-only or open a copy of the document for editing."

I have never locked any of my files.

WHAT CAN I DO?
HELP!!!

What is a good alternative to LibreOffice?

This appears to be a perfect duplicate of Q55755. If so, and the answer at Q55755 does NOT help then I suggest filing a bug-report quoting both this & the other question.

See also Q56101 for a fix.