An issue has been discovered in the current Windows Live Messenger beta that may leave users unable to sign-in. Only a very small number of users may encounter this issue but wanted to proactively message this to the community. To avoid the issue yourself, please DO NOT block a group and then leave the group. If you plan to leave a group, please unblock it first. Following this precaution will ensure that you will not be affected by this issue.
We are currently working hard to fix the problem and allow those customers who have been affected to once again sign-in to the Messenger beta, reports Messenger team blog.
So, If you are using the Messenger beta take this to heart: if you want to leave a Group, make sure it isn’t blocked first!