In kirby 2.5.7 the panel does not work on shared servers of german provider strato. This fix makes it work

gistfile1.txt

open file kirby/vendor/getkirby/toolkit/lib/url.php
change this (around line 397):

-----------------------------------------------------------------------------
public static function unIdn($url) {

    if(!function_exists('idn_to_ascii')) return $url;

    // disassemble the URL, convert the domain name and reassemble
    $variant = defined('INTL_IDNA_VARIANT_UTS46') ? INTL_IDNA_VARIANT_UTS46 : INTL_IDNA_VARIANT_2003;
    $host = idn_to_ascii(static::host($url), 0, $variant);
    if($host === false) return $url;
    $url  = static::build(['host' => $host], $url);

    return $url;
  }
-----------------------------------------------------------------------------

to that:

-----------------------------------------------------------------------------
  public static function unIdn($url) {

    if(!function_exists('idn_to_ascii')) return $url;

    // disassemble the URL, convert the domain name and reassemble
    $host = idn_to_ascii(static::host($url));
    $url  = static::build(['host' => $host], $url);

    return $url;
  }
-----------------------------------------------------------------------------

Now you will be able to access the panel. But you'll see a message there (at least with debugging switched on):

  idn_to_utf8() expects at most 2 parameters, 3 given
  
To get rid of the message change this (around line 378)

-----------------------------------------------------------------------------
  public static function idn($url) {

    if(!function_exists('idn_to_utf8')) return $url;

    // disassemble the URL, convert the domain name and reassemble
    $variant = defined('INTL_IDNA_VARIANT_UTS46') ? INTL_IDNA_VARIANT_UTS46 : INTL_IDNA_VARIANT_2003;
    $host = idn_to_utf8(static::host($url), 0, $variant);
    if($host === false) return $url;
    $url  = static::build(['host' => $host], $url);

    return $url;

  }
-----------------------------------------------------------------------------

to that:

-----------------------------------------------------------------------------
  public static function idn($url) {

    if(!function_exists('idn_to_utf8')) return $url;

    // disassemble the URL, convert the domain name and reassemble
    $host = idn_to_utf8(static::host($url));
    $url  = static::build(['host' => $host], $url);

    return $url;

  }
-----------------------------------------------------------------------------  
  
The code you put in derives from kirby 2.5.5 and kirby 2.5.7 will work as expected afterwards.
This solution was brought to me from texnixe (https://github.com/texnixe).
Thank you texnixe!

I posted it as a gist, because up to now you don't find anything when you search github for "kirby panel strato"