I have been running BackupPC 3.3.0 for a few years without trouble. After upgrading to 4.1.1, I am receiving the following errors on backups -

rsync error: rsync failed on file IO(code 11) at exclude.c(1058) [client 3.0.9.6] 

** this error is from backup of OpenSuSE Leap42.1 systems with rsync 3.1.0 **

rsync error: unexplained error(code 255) at io.c(629) [receiver 3.0.9.6] 

** this error is from backup of Win10 system with cygwin64 rsync 3.1.2 **

 Nothing was changed in the hosts configurations.

Let me know if configuration files are needed.

Log entries -

server log -

2017-04-08 06:00:01 Started incr backup on k1pdr (pid=2295, share=/)

2017-04-08 06:00:01 Started incr backup on lpdr (pid=2298, share=/cygdrive/c)
2017-04-08 06:00:09 Backup failed on k1pdr (rsync error: error in file IO (code 11) at exclude.c(1058) [client=3.0.9.6])
2017-04-08 06:00:09 Backup failed on lpdr (rsync error: unexplained error (code 255) at io.c(629) [Receiver=3.0.9.6])

k1pdr log - 
2017-04-08 06:00:01 Renaming /BackupPC/pc/k1pdr/XferLOG.698.z -> /BackupPC/pc/k1pdr/XferLOG.698.z.tmp
2017-04-08 06:00:01 incr backup started for directory /
2017-04-08 06:00:02 Got fatal error during xfer (rsync error: error in file IO (code 11) at exclude.c(1058) [client=3.0.9.6])
2017-04-08 06:00:07 Backup aborted (rsync error: error in file IO (code 11) at exclude.c(1058) [client=3.0.9.6])

lpdr log -
2017-04-08 06:00:01 Renaming /BackupPC/pc/lpdr/XferLOG.607.z -> /BackupPC/pc/lpdr/XferLOG.607.z.tmp
2017-04-08 06:00:01 incr backup started for directory /cygdrive/c
2017-04-08 06:00:02 Got fatal error during xfer (rsync error: unexplained error (code 255) at io.c(629) [Receiver=3.0.9.6])
2017-04-08 06:00:07 Backup aborted (rsync error: unexplained error (code 255) at io.c(629) [Receiver=3.0.9.6])