Updated Roundcube version uses identities to send mail from.
If you see the sender field is blank, you will need to add an identity. To fix this, please do the following:
1) In Roundcube go to Settings
2) Click the Identities tab
3) Click "New Identity"
4) Fill out the information on the right side form and submit.
You can create multiple identities if you send from multiple addresses within the same mailbox. When sending mail, just select the one to send from if you have multiple.