If you happen to run a vBulletin forum and hit an issue where you cannot login to the site without first selecting the "Remember Me" checkbox, would you happen to be on CloudFlare or be using a reverse proxy? Make sure for CloudFlare you have the list of their upstream proxy IPs up to date or your REMOTE_ADDR of the client IP might not be getting set correctly.
You can find the latest list of CloudFlare proxy IPs here:
Do not trust the sample configs found on the CloudFlare site. They are almost always out of date.