Can't login after upgrade to php5.3

ZC1.3.9h - Hostagator VPS - Hostgator upgraded us to php5.3 last night. Everything fine on the outside but trying to log into admin gets me: "There was a security error when trying to login."

Ok what have you tried?

1. Have you cleared all browser cookies/cache on your computer, or tried with a different browser
2. Are there any error messages appearing in the zencart error cache on your site
3. What mods do you have on yoursite, as not all mods are php 5.3 compatible

Thank you for the suggestions. It did turn out to be about the cookies. I just received this email from hostgator support:

"I was able to track down this issue to the new default php.ini file which has the following variable set:
session.use_only_cookies = 1

This was causing a session conflict, since zen cart was using the database for session management. I changed the above value to "0" and the admin panel is now loading."

Problem solved.