launchd AppleScript error "Folder some object not found"

Question

I have no specific explanation for your symptoms, but a few observations:

Your script as written doesn't work on my OS X 10.8 and 10.9 machines - find a streamlined version below. What version are you using?
Given the nature of your script there's no strict reason to save it as an application - you could instead save it to a regular *.scpt file and tell Lingon to execute the following (note the need to use full paths both for osascript and your *.scpt file):
/usr/bin/osascript /path/to/your/script.scpt

The problems I encountered on OS X 10.8 and 10.9:

The mount volume command neither directly returns a value nor sets the global result variable - my streamlined version simply tries instead to access the share as a mounted disk.
The variable name server resulted in strange errors; changing the name made them go away.

Streamlined version:

# Mount the share.
# Note that at least on OS X 10.8 and 10.9 this command:
#  - is a no-op if the share is already mounted
#  - in either case has NO return value and does NOT set the 
# global `result` variable.
# The share will be mounted as "disk" /Volumes/{shareName} (POSIX) / 
# {shareName}: (HFS)
# Note that the `mount volume` is defined in the Standard Additions dictionary, 
# not the Finder's.
set destShareName to "sharename"
mount volume "smb://<username>:<password>@server.domain.com/" & destShareName

# Getting here without error means that the share was just mounted
# or was previously mounted.

tell application "Finder"

    # Get a reference to the mounted share.
    # !! It seems that at least on OSX 10.9 it can take a few seconds 
    # !! before a just-mounted
    # !! share becomes accessible.
    # !! We try for a while, but eventually give up.
    set numTries to 5 # How many seconds (at least) to keep trying.
    set numTry to 1
    repeat
        try
            set destShare to disk destShareName
            exit repeat
        on error errMsg number errNo
            set numTry to numTry + 1
            if numTry > numTries then error errMsg & " - giving up after " & numTries & "+ seconds." number errNo
            delay 1 # Sleep for 1 second, then try again.
        end try
    end repeat

    # Define the SOURCE *HFS PATHS*.
    set sourceFolderPaths to {"Macintosh HD:<location to share1>", "Macintosh HD:<location to share2>", "Macintosh HD:<location to share3>", "Macintosh HD:<location to share4>"}
    # Define the corresponding DESTINATION *FOLDER NAMES*
    set destFolderNames to {"share1", "share2", "share3", "share4"}

    # Process all folders.  
    repeat with i from 1 to length of sourceFolderPaths
        set sourceFolderPath to item i of sourceFolderPaths
        set destFolderName to item i of destFolderNames
        # Get a reference to the destination folder or create it, if it doesn't yet exist.
        try
            set destFolder to folder destFolderName of destShare
        on error # folder access failed -> assume it must be created
            set destFolder to make new folder with properties {name:destFolderName} at destShare
        end try
        # Copy the files. This will overwrite all files.
        duplicate items of folder sourceFolderPath to destFolder with replacing
    end repeat

    # Unmount the share.
    eject destShare

end tell