Well, I got the sense that this had to be on my computer. I finally figured out that when I tracked a package yesterday, I somehow got a program that claims to be a package tracking program. (I did not want it--it was one of those things you somehow get without realizing it.) I got rid of the program, rebooted, and everything seems fine now. I am surprised Malwarebytes did not spot it for me.
It is very curious that it absolutely only worked on links from ScubaBoard.
It is very curious that it absolutely only worked on links from ScubaBoard.