Solved: I decided that maybe one thing to try was to see if the router (DLink EBR2310) was malfunctioning. So I was preparing to remove it and put the ethernet cable directly into the computer. But then as I was doing this I noticed the reset button on the bottom of the router. I thought, it's easier to try a reset first, and if that doesn't work, then change the cables. But, yes, after holding the button down a few seconds, and then testing a couple of the 401 blocked websites, the problem was solved.
No idea what happened to the router, it's been reliable in the past.