Same problem here since about 1 month ago. Maybe a firmware update. Previously it worked flawlessly, now many times I get a timeout error when connecting my account to youtube. Also, many videos are not loaded, just the youtube logo appears after a few seconds trying to load the video. Maybe this is just HD videos, but I'm not sure. My network seems ok as I don't have problems in any other device (PC or tablet or even SOny bluray player).

My bravia is a KDL40EX520. Firmware is version PKG4.005AAA (latest as of today).

BTW, I'm also experiencing problems with "Bravia Internet Video". Most of the time I cant load any of those channels. Problems appeared about the same time.

Another curiousity is that my Sony BluRay player has no problems connecting to youtube or bravia internet video. So it seems a problem with the TV and it's latest firmware.