If you are used while loop you needed to input inc %i see this code fallowing:

alias mirccc2html {
  var %stdin = $+($replace($mid($1,$calc($pos($1,:,2) + 1)),$chr(32),$lf),$chr(15)), %stdout, %LF = 0, %i = 1, %len = $len(%stdin)
  while (%i <= %len) {
    var %char = $mid(%stdin,%i,1), %i = %i + 1
    if (%char == $lf) var %LF = %LF + 1, %stdout = $+(%stdout,$iif(2 // %LF,&nbsp;,$lf))
    elseif (%char == $chr(2)) var %^B = $iif(%^B,$false,$true), %stdout = $+(%stdout,$iif(%^B,<b>,</b>))
    elseif (%char == $chr(29)) var %^I = $iif(%^I,$false,$true), %stdout = $+(%stdout,$iif(%^I,<i>,</i>))
    elseif (%char == $chr(31)) var %^U = $iif(%^U,$false,$true), %stdout = $+(%stdout,$iif(%^U,<u>,</u>))
    elseif (%char == $chr(22)) var %^R = $iif(%^R,$false,$true), %stdout = $+(%stdout,$iif(%^R,<span class="R">,</span>))
    elseif (%char == $chr(3)) {
      if (%^K) var %^K = $false, %stdout = $+(%stdout,</span>), %color, %background-color
      if ($mid(%stdin,%i,1) isnum) var %color = $base($v1,10,10,2), %i = %i + 1
      if ($mid(%stdin,%i,1) isnum) var %color = $base($+(%color,$v1),10,10,2), %i = %i + 1
      if ($mid(%stdin,%i,1) == ,) {
        var %i = %i + 1
        if ($mid(%stdin,%i,1) isnum) var %background-color = $base($v1,10,10,2), %i = %i + 1
        if ($mid(%stdin,%i,1) isnum) var %background-color = $base($+(%background-color,$v1),10,10,2), %i = %i + 1
      }
      if (%color) var %^K = $true, %stdout = $+(%stdout,<span class=" $+ $+(K,%color,%background-color) $+ ">)
    }
    elseif (%char == $chr(15)) {
      if (%^B) var %^B = $false, %stdout = $+(%stdout,</b>)
      if (%^I) var %^I = $false, %stdout = $+(%stdout,</i>)
      if (%^U) var %^U = $false, %stdout = $+(%stdout,</u>)
      if (%^R) var %^R = $false, %stdout = $+(%stdout,</span>)
      if (%^K) var %^K = $false, %stdout = $+(%stdout,</span>), %color, %background-color
    }
    else {
      var %LF = 0
      if (%char == &) var %stdout = $+(%stdout,&amp;)
      elseif (%char == <) var %stdout = $+(%stdout,&lt;)
      elseif (%char == >) var %stdout = $+(%stdout,&gt;)
      else var %stdout = $+(%stdout,%char)
    }
    inc %i
  }
  return $replace(%stdout,$lf,$chr(32))
}

if you forget this (inc %i) this script will have problem.