The exception error means that the connection has been closed (either by the POP3 server itself, or by some ill-behaved network device along the way).
Do you have access to the POP3 server? If yes, please try checking the POP3 server log and see whether and why the server has closed the active connection.
Workaround in Rebex components could be to retry the GetMessage operation a few times if this particular error is caught in a try catch block.
Most full-featured email clients, retry operations a few times before giving up. This way they eliminate network/server glitches.