The problem was a result of me taking a portion of my expanded script (supports 9 pastebins + more options). I basically opened the "pastecontrol_pasthis" socket but tried receiving events for "pastecontrol.pasthis" (. instead of _).

Here's the fix:

Code:
alias -l makepoststring { 
  bset -t &poststring 1 $1
  var %line = 1, %lines = $cb(0)
  while (%line <= %lines) {
    bset -t &poststring $calc($bvar(&poststring,0) + 1) $+($urlencode($iif($cb(%line) != $null,$v1,$chr(32))),%,0A)
    inc %line
  }
}
alias -l paste {
  if (!$0) { return $false }
  if (!%pastecontrol.lines) { %pastecontrol.lines = 5 }
  if ($cb(0) > %pastecontrol.lines) { 
    if (!%pastecontrol.website) { %pastecontrol.website = pasthis.com }
    pasteweb %pastecontrol.website /msg $1
  }
  else { msg $1- }
}
alias -l pasteweb {
  if ($1 == pasthis.com) {
    if ($sock(pastecontrol.pasthis)) { return $false }
    sockopen pastecontrol.pasthis pasthis.com 80
    sockmark pastecontrol.pasthis $2-
  }
}
alias -l urlencode { return $regsubex($1,/([^A-Z0-9_\-])/gi,% $+ $base($asc(\t),10,16,2)) }
alias -l websites { return pasthis.com }

#pastecontrol on
on *:input:*:{
  if ($inpaste) {
    .timer 1 0 paste $target $1-
    haltdef
  }
}
#pastecontrol end

on *:sockopen:pastecontrol.pasthis:{
  if ($sockerr) { return }
  makepoststring $+(lifetime=max&private=0&title=Autopaste%20by%20,$me,&nickname=,$me,&text=)
  sockwrite -n $sockname POST /mirc/ HTTP/1.1
  sockwrite -n $sockname Host: pasthis.com
  sockwrite -n $sockname Accept: */*
  sockwrite -n $sockname Connection: Close
  sockwrite -n $sockname Content-Type: application/x-www-form-urlencoded
  sockwrite -n $sockname Content-Length: $bvar(&poststring,0)
  sockwrite -n $sockname
  sockwrite $sockname &poststring
}
on *:sockread:pastecontrol.pasthis:{
  var %data
  sockread %data
  if (Pasthis-URL: * iswm %data) {
    if ($sock($sockname).mark != $null) { $v1 $gettok(%data,2,32) }
  }
  elseif (Pasthis-Message: * iswm %data) { echo -a * Paste control: $gettok(%data,2-,32) }
}

dialog pastecontrol {
  title "Paste Control"
  size -1 -1 180 162
  option dbu notheme
  check "Enable paste control", 1, 4 5 60 8
  text "Only paste to the web when clipboard contains more than ", 2, 4 25 139 8
  edit "", 3, 145 23 17 11, number
  text "lines", 4, 164 25 11 8
  text "I want paste control to paste to the following website:", 5, 5 41 132 8
  list 6, 4 53 133 80, radio size vsbar
  button "Ok", 7, 98 146 24 12, ok
  button "Cancel", 8, 124 146 24 12, cancel
  button "Apply", 9, 150 146 24 12, disable
}
on *:dialog:pastecontrol:init:*:{
  if (!%pastecontrol.lines) { %pastecontrol.lines = 5 }
  if (!%pastecontrol.website) { %pastecontrol.website = pasthis.com }
  if ($group(#pastecontrol).status == on) { did -c $dname 1 }
  did -a $dname 3 %pastecontrol.lines
  didtok $dname 6 32 $websites
  if ($didwm($dname,6,%pastecontrol.website,1)) { did -s $dname 6 $v1 }
}
on *:dialog:pastecontrol:sclick:*:{
  if ($istok(1 6,$did,32)) { did -e $dname 9 }
  elseif ($istok(7 9,$did,32)) {
    $iif($did(1).state,.enable,.disable) #pastecontrol
    %pastecontrol.lines = $did(3)
    %pastecontrol.website = $did(6).seltext
    if ($did == 9) { did -b $dname 9 }
  }
}
on *:dialog:pastecontrol:edit:3:{ did -e $dname 9 }

menu channel,menubar,query,status {
  $iif(!$dialog(pastecontrol),Paste control): dialog -m pastecontrol pastecontrol
}


This also fixes the /msg error. I forgot to pass the text pasted to the /paste alias.