If this error comes up every time you check mail than it is likely that you have two accounts setup that point to the same POP mailbox inside Accounts Setup; you can prevent this by going to the > Network Settings> tab of the "Program Options" window (Shift+F7) and setting the maximum number of threads to 1.

If this error comes up from time to time it could mean that Poco encounters an error while downloading your mail and it fails; then POP mailbox would remain locked until the POP server timeouts (usually a minute or two), so it would not allow access during that time, you would have to wait a couple of minutes and then try again to check mail.

Comment from Michael
In this last case is the problem may be related to the frequency with which you have set Poco to automatically check for new mail. What you can do to get around it (or at least to reduce it's frequency) is to increase the interval at which Poco automatically checks for mail. To do this open the program settings by pressing Shift+F7, then press the "Checking Mail" tab and increase the setting for "Check for new mail every (minutes):" setting. (I'm assuming you have the box beside this checked).

Downloading and installing PocoBot will also solve this problem in that it won't display error messages but rather flash a red icon in the system tray when it's having problems - much less intrusive than the Poco error window.

For more information see the article on Timeouts.