Hello
those EIO errors have been added in some of the more recent cvmfs version.
with cvmfs_talk -i <repo> internal affairs you get a better description of where those EIO error happened.
It is true that this is not very nice that there is just a number and not more descriptive explanation in the syslog message.
"eio.01",
"EIO returned to calling process. cvmfs.cc:cvmfs_lookup()"
so the lookup failed because you were in offline mode and the file was not in the local cache.