The problem is gone, I think. At least, I haven't had to log in recently. :)
I was using horizons.acornhosting.net and would have had an acornhosting.net cookie. So that might be it.
Another thing that I changed about the same point is that I switched the server name specificied in config.tcl to match the one in the kernel (canonical server name?).
Not sure which of those did it, but it might be useful info for someone else.
