The issue occurs if the learner/ learners who have logged in and are facing session expired issue. In such cases, the LF login, as well as all the learners, faces sudden logout issues.
Please check if any learner is facing session expired issue.
To solve this issue kindly refer this link: http://eraqa.mkcl.org/?qa=4335/facing-session-expired-error-after-era-login
In case you are still facing the same issue, kindly request you to generate a SOLAR support ticket.