What does the error message POP EOF or I/O error mean?

EOF or I/O error almost always means EOF. That is, the network connection with the client dropped unexpectedly. At the point where this message is issued, Qpopper no longer knows if it was an EOF or an actual I/O error, and so it reports the error number just in case it really is an I/O error (which it almost never is). When it is just an EOF, the associated error is meaningless.