I spoke to Danny about this today.
"Server too busy" is just that. It mostly occurs before during and after the lunchtime period, although I noticed a couple of times this morning around 0930. The server, which was replaced some while ago is need of an upgrade - yet again. The consequences of a successful website.
Danny is aware, and he is on the case.
One little tip. It might take a couple of goes, but just go right back in - it often opens again.