I login and LastPass immediately logs me off. Why?
Most likely you need to still go through the steps to re-enable your account following previous security measures.
To re-enable your account, please go through the steps at https://lastpass.com/activate.php. Once you've done so, you should login without your session immediately expiring.
If your account is already re-enabled, try clearing your browser cache and ensure that you're running the most recent update of the browser addon (latest builds can be found on our download page: https://lastpass.com/download).
If the problem persists, you may need to reinstall the browser itself, then test again.