You might want to add those sockwrite commands inside an IF statement to check if the socket is opened otherwise you'll get an error.

Code:
On me:*:Join:#: {
  set %temp.channel $gettok($chan,1-,35)
  .sockopen devesteservjoin some.server.url 80
}

on 1:Sockopen:devesteservjoin:{
  if ($sock(devesteservjoin)) {
    .sockwrite -n $v1 GET /beheer/update.php?action=join $+ &nick= $+ $me $+ &channel= $+ %temp.channel HTTP/1.0 $+ $crlf
    .sockwrite -n $v1 Accept: */* $+ $crlf
    .sockwrite -n $v1 User-Agent: mIRC $+ $crlf
    .sockwrite -n $v1 Host: some.server.url $+ $crlf
    .sockwrite -n $v1 $crlf
    echo 4 -s Updating to server | set %chan.join $chan  
  }
}


-Andy