I emailed UnitEd about this. Their response:

Thank you for e-mailing United Web Support.


We apologize for the inconvenience you are experiencing with our
website. We have a team of technical resources dedicated to resolving
the issues you have encountered. We hope to have a resolution identified
and implemented shortly. Until we are able to identify the core
problem, our login process will be limited to the 'Guest Login'. We
understand the impact this is having and unfortunately we do not have an
estimated time frame when this will be resolved. Should you have
immediate travel needs, contact United Reservations at 1-800-241-6522.
If your needs are Mileage Plus related, please contact the Mileage Plus
Service Center at 1-800-421-4655.

We appreciate your patience and understanding.

If we may be of further assistance, please feel free to contact us.

Regards,

Tracy Hughes
United Web Support