Just in case someone else has the same problem, here is how to solve it.
ACP > Settings > Options >Site Name / URL / Contact details
now
Scroll down to "Redirect Domain Whitelist" and add the various way people log into the site. Again, I'm not sure if all of these are needed, but they fixed my problem.
nxxxxxxxxxs.com
nxxxxxxxxxs.com/forum
www.nxxxxxxxxxs.com
www.nxxxxxxxxxs.com/forum
http://nxxxxxxxxxs.com
http://nxxxxxxxxxs.com/forum
http://www.nxxxxxxxxxs.com
http://www.nxxxxxxxxxs.com/forum
Hopes this helps someone....