Your tampering with kap's IF logic means that the first 4 lines will not match the times 05:59:59 11:59:59 17:59:59 23:59:59
Also, you should probably list the channels where this greetings is sent, because most large channels will not want this

Code
on !*:JOIN:#channelA,#channelB:{
  if     ($time < 06) var %ret Goedenacht
  elseif ($time < 12) var %ret Goedenmorgen
  elseif ($time < 18) var %ret Goedenmiddag
  else                var %ret Goedenavond
  msg $chan %ret $nick $+ !
}