Mavericks: Unable to load plist. Launchctl says "Invalid property list", tried plutil, SIP, permissions


I want to use the following list to run a backup script on macos High Sierra.

File name:
com.pollux.backup.plist

Content:




  
    Label
    com.pollux.backup.plist
    ProgramArguments
    
     sh
     -c
     /bin/backup_pollux_server
     KeepAlive
     
     RunAtLoad
     
     StartInterval
     3600
     StandardErrorPath
     /tmp/pollux_backup.err
     StandardOutPath
     /tmp/pollux_backup.out
  

Permissions: -rw-r--r--@ 1 root wheel 694 3 Apr 19:57 com.pollux.backup.plist

I use the same script and plist to back up my wife's MacBook (Mavericks). It works perfectly. But when I try to load it with launchctl on my MacBook, I get the following error message:

/Library/LaunchDaemons/com.pollux.backup.plist: Invalid property list

I checked the plist with plutil:

LaunchDaemons pollux$ plutil com.pollux.backup.plist
com.pollux.backup.plist: OK

xmllint only displays the content of the plist.

I turned off the SIP:

LaunchDaemons pollux$ csrutil status
System Integrity Protection status: disabled.

Does anyone have an idea of ​​what to try next?