I think you're misunderstanding my multi user statement. By multi user I mean if the first word matches more than one user: like Nill matches Nillen, Nillens and Nillensbot.

That code should work, but try this code as well, I updated the $fline code to make sure it actually hits a target. It should be a ton faster in big channels.
Code:
on *:INPUT:#: {
  if (%multi) {
    var %f $fline(#,$1 $+ *,0,1).text
    if (%f) {
      var %true 1
      while (%f) {
        var %n $fline(#,$1 $+ *,%f,1).text
        msg # $effect(%n) $2- 
        dec %f
      }
    }
  }
  else { 
    var %n $fline(#,$1 $+ *,1,1).text
    if (%n) { 
      var %true 1
      msg # $effect(%n) $2-
    }
  }
  if (%true) halt
}
menu channel { 
  $iif(%multi,$style(1)) Multi user : $iif(%multi,unset %multi,set %multi 1)
}
alias -l effect return 6¤3.11·13´6`9·4.13¤ $+ 1 $1 7¤3.11·13´6`9·4.11¤1

Works fine for me:
Quote:
<@nillens> Hello there, how are you?
<@nillens> 6¤3.11·13´6`9·4.13¤1 nillensbot 7¤3.11·13´6`9·4.11¤1 are you there?
<@nillens> 6¤3.11·13´6`9·4.13¤1 nillens 7¤3.11·13´6`9·4.11¤1 are you there?
<@nillens> Hello?
<@nillens> 6¤3.11·13´6`9·4.13¤1 nillensbot 7¤3.11·13´6`9·4.11¤1 ???
<@nillens> 6¤3.11·13´6`9·4.13¤1 nillens 7¤3.11·13´6`9·4.11¤1 ???


Nillens @ irc.twitch.tv
Nillen @ irc.rizon.net