Update: The outage appears to be resolved and Twitter users can once again log into the desktop site.
A Twitter outage has logged many users out of the website and prevents them from logging back into the site.
At approximately 3:45 PM ET, Twitter users worldwide reported they were suddenly logged out of the desktop website, with thousands reporting issues on DownDetector.
Source: DownDetector
When attempting to log back into the site, the logins would appear to work, but the site would redirect you to the logout URL.
This appears to primarily be affecting users on Google Chrome and Chromebook, with my account logged out in Chrome but not in Mozilla Firefox.
After attempting to log back into Twitter with Google Chrome a second time, the site prompted me for additional information, stating that the account had unusual activity.
Once I entered that information, I could log in to the account again.
This does not appear to be the same for other people affected by the repeated cycle of login and logouts.
No issues are listed on Twitter’s status page or support account, so we will likely have to wait for the outage to resolve itself.
The issues resolved without any updates from Twitter on the cause.
Source: www.bleepingcomputer.com