Oddly enough, I had this exact problem last night and I DIDN'T upgrade to X.11.4 yet. When I went to my account, it asked for my login, so I typed in my usual U_N@mac.com and P/W and it kept failing. I ended up changing my U/N to U_N@ME.com and it finally logged in and worked fine. My .Mac address has always worked fine up until now.