I had the same problem and finally resolved it by clearing the website data for MR. The next time I logged out/in, Safari asked for permission to use the data.
Have you checked the password tab of Safari preferences for an entry for MR?
When I enter the password to get into the macrumors forum, Safari never asks me to remember the password. Whats going on?
Here is what you do. Log out of MR, then restart your browser and go to the MR login page and enter your username and password, but do not click login yet. Now hit command-r to refresh the page and you will get a prompt to save the password to Keychain.
Wow that is like magic. How did you figure that one out? The steps didn't make any sense but it works!!!