This instance seems to be experiencing some instability. I've received 502 Bad Gateway and 503 Service Unavailable errors when attempting to login at times recently.
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@neildarlow Try clearing the firefox cache; (Edit->Settings search for cache); it often seems to get confused and get fixed by that (similarly it shows the wrong images sometimes)
@penguin42 I'm using the Tootle client. It's been completely stable until today. The errors received are HTTP statuses from the remote end.