I have entered the correct data for my Web.de account, but I cannot retrieve any emails

Make sure that you have installed the SSL patch.

Make sure that reception for POP3/IMAP is activated in your account. By default, Web.de prohibits the retrieval of emails via POP3/IMAP. To do this, log in to your Web.de account via the Web.de homepage. Then click on "Inbox" -> "Settings" -> "POP3/IMAP retrieval" -> "Sending and receiving emails via an external programme (Outlook, Thunderbird)" (as at 07/01/2016).

By default, Web.de only retrieves the "Friends and acquaintances" folder via POP. Either add the sender address to the Web.de address book, or select "Inbox" -> "Folder overview" -> Move the mouse over the "Unknown" folder, click on the spanner that appears on the right and tick the box "Retrieve emails in this folder via POP3", then the emails will also be retrieved via POP (as of 07.01.2016).