Craig White:
try adding this to /etc/sysctl.conf
# hack to fix problems with internet routers net.ipv4.tcp_window_scaling = 0
Jacques B.:
That did it!! Thanks to all for your input. Special thanks to Craig and Robert for the solution!
If that hadn't been it, the other thing I would have suggested, and is probably worth remembering for the future, is whether your browsers are set up to verify revocation of SSL certificates. That often fails, and without giving you any clues as to what's going wrong.