No, something else is wrong. Here are the script and log result of
this script run in SE.
By the way, OS X 10.4.8, SE Version 2.1.1 (81), AS 1.10.7
Your script errors at "log myReply" when run using Script Debugger,
the Apple Event Log is:
-- started
tell application "Mail"
activate
get message viewer 1
--> message viewer id 4781360
get selected messages of message viewer id 4781360
--> {
message id 93784 of mailbox "AppleScript"
}
reply message id 93784 of mailbox "AppleScript" with opening window
end tell
-- stopped
Am also using 10.4.8, Mail 2.1.1 (752.3), AS 1.10.7
