Why does Webmail load into another page that is not either RoundCube or Horde?
If there is another page that is starred for an email account, then it will load this page first. There are various pages that the account can load first such as Email Filters, Autoresponders, Calendars, etc. Ensure that when you access Webmail for the account, that the starred option is either RoundCube or Horde such that either one of these interfaces is where the account will load into first.
These options are found on the top right of the main webmail page. Select your email address, and a dropdown menu will appear with your options.