Sometimes RoundCube will no longer allow a user to access their webmail, it will show either the symptoms of a failed login or redirect to a login page if accessed from cPanel. Below shows you how to correct this issue should it happen.
In the folder
Look for the file called <email_user>.rcube.db
Rename the file to <email_user>.rcube.db.bak or move it out of the folder
Login to the account using either cPanel or https://www.domain.com:2096 and select RoundCube.
Your account should start working again.