Roundcube: "Mailbox does not exist/must subscribe"

Sooo, I managed to ftp over the Maildirs from my boss's old hosting provider and copy over what needs to be copied. Everyone's stuff seems to be there, but there's a problem:

When clicking on any folder other than the Inbox, Sent, Drafts, or Trash, the message "Mailbox does not exist or must be subscribed to" appears. Is there any way to fix this? I found that by going to Settings –> Special Folders --> Folders tab and clicking the + and typing in the folder name, this fixes it…for one folder. One of the agents has over 100 subfolders.

What do I do?