Same here. I loaded a save I had on the section where the MC says he has to remember to take the keycard back with him, and after that, it immediately jumps back to the main menu screen. I even tried loading a save I had from just before the computer riddle starts, repeating the riddle section again, with the same result. I'm not going to bother starting from the beginning again, so the dev really needs to get their crap together to find a solution/workaround (assuming they actually released new content after this quest).
The "last" quest involves you (the MC) sneaking into a building to shut down the power grid by using a computer. This computer provides riddles in the form of solving 3-4 reverse/transform ciphers (by following some simple puzzle rules) to initiate the grid shutdown. However, after this riddle-solving, the game seems to abruptly end for some reason.