For some time now, Minecraft Launcher couldnt login (“cant connect to minecraft.net”).
This happened only on one machine.
Enter wireshark:

Alert (Level: Fatal, Description: Certificate Unknown)

Wtf? It works everywhere!
Then it struck me. The BIOS clock resets every time I start the Windows box, because the mobo’s battery died.
I usually dont care, because I use Windows for gaming only, and then time is irrelevant ;P

So I set the correct time, and voila… the certificate becomes valid. Login works. Rah!