Got it worked out. There were a number of things I was doing incorrectly actually, mainly how I was attempting to target users using _AD_FQDNToSamAccountName when I'd already gotten their SamAccountname.
Finished script:
#include <AD.au3>
; Open Connection to the Active Directory
_AD_Open("Administrator", "password")
If @error Then Exit MsgBox(16, "Active Directory Master Password Reset", "Function _AD_Open encountered a problem. @error = " & @error & ", @extended = " & @extended)
Global $iReply = MsgBox(308, "Active Directory Master Password Reset", "This script changes the password for all Users in the 'Quality' OU." & @CRLF & @CRLF & _
"Are you sure you want to change the Active Directory?")
If $iReply <> 6 Then Exit
Global $aUsers
;update target to the appropriate OU you wish to reset
Global $sOU = "OU=Users,OU=Quality,OU=Engineering,DC=dev,DC=fbf"
$aUsers = _AD_GetObjectsInOU($sOU, "(objectclass=user)", 2, "samaccountname")
;reset password loop
FileWriteLine("log.txt", @Hour & ":" & @Min & ":" & @Sec & " - " & "Beginning Password Reset..." )
For $i = 1 to $aUsers[0]
Global $iValue = _AD_SetPassword( $aUsers[$i], "demo") ; set password here
If @error Then Exit FileWriteLine("log.txt", @Hour & ":" & @Min & ":" & @Sec & " - " "There was an issue changing the password for " & $aUsers[$i] & " - @error = " & @error & ", @extended = " & @extended)
FileWriteLine("log.txt", @Hour & ":" & @Min & ":" & @Sec & " - " & $aUsers[$i] & "'s password changed to 'demo'" )
Next
FileWriteLine("log.txt", @Hour & ":" & @Min & ":" & @Sec & " - " & "Ending Password Reset" )
Global $iReply = MsgBox(0, "Active Directory Master Password Reset", "Password Reset Successful")
_AD_Close()