PuTTY fatal error: Server refused to start a shell/command

The problem was a shortage of RAM.

It sounds like something in your putty configs is requesting a specific shell after login that is no longer approved. Try creating a new session instead of your saved one and input as few settings as possible, just the host and any authentication details. That should get you in. Then if you want to fix your previous saved session look through the settings for bits about remote command, preferred shell, etc.