rsync OSX-linux problem

Hi,

I have an rsync daemon on a MAC-OSX srv.  The daemon share a read-only folder.

On a linux "stand-by" server (no user activity here) I use rsync client to
replicate from server.

On about 85000 files I have 4 error like this:
-------------------------
rsync: delete_file: unlink "/var/bck/srv_kerio/store/mail/xxxx.it/pppp.qqqq/INBOX/
#msgs/00004746.eml" failed: Read-only file system (30)
-------------------------

This file not exists on the rsync-server and the local filesystem on the stand-by server is NOT in read-only mode.

Any suggests?

TIA