rsync occassionally issues the message "rsync error: unexplained error (code 255) at main.c(1506) [generator=3.0.4]", apparently as it is performing clean up of child processes. Does this message indicate an issue with the transfer? Is there a known condition that causes it?
I should mention that we are using rsync in conjunction with ssh. Please let me know if there are additional details you would like.
If rsync starts a child process, it can set its exit code based on what that process returned. If that process didn't exit cleanly, the code can be out of the normal list of error codes that rsync returns, which generated the unexplained error.
Note that this question would have been better asked on the mailing list, since it is not noting a bug.