Thank you for this bug report. To properly diagnose the problem, we
need a short but complete example script to be able to reproduce
this bug ourselves. 

A proper reproducing script starts with <?php and ends with ?>,
is max. 10-20 lines long and does not require any external 
resources such as databases, etc.

If possible, make the script source available online and provide
an URL to it here. Try to avoid embedding huge scripts into the report.

This is the code:
<?php
  $filename = '../images/products/thumbnails/101D0-86_jpg_thumb_75x75.jpg';
  echo 'File does exist but it is not found: ' . $filename . '<br />';
  echo 'If you copy the destination in a browser the file is visible<br />';
	if (file_exists($filename))
    echo 'YES<br />';
  else
    echo 'NO<br /><br />';

  $filename1 = '../images/products/thumbnails/tmp_gif';
  echo 'File (in the same directory)) does exist and it is found: ' . $filename1 . '<br />';
	if (file_exists('../images/products/thumbnails/tmp.gif'))
    echo 'YES<br />';
  else
    echo 'NO<br />';
?>

This is the online link:
http://81.174.30.88/tmp/tmp.php

"A reproduce script" means I need a script that I can copy/paste and reproduce the problem. Probably, with some additional actions (like creating a certain file etc.).

How do you suppose I should reproduce the problem with the script you've given? Obviously I don't have all your files here.

Please re-read my last post.
And don't paste huge scripts not related to the reproduce code if not asked for.
Basically what I need to reproduce it:
1) permissions on the files you can't find (I'm sure the filetype and the way you create them doesn't matter).
short version:
do `ls -l <file>` and paste the result here.

2) a *short* but *complete* reproduce case I can copy/paste and test it.
most likely I'll need to know your directory structure too (because you refuse to use absolute paths).

This is the ls -l tmp.gif:
-rw-r--r--  1 XXXXX psacln 0 Oct 12 22:16 tmp.gif

This is the ls -l 101D0-86_jpg_thumb_200x200.jpg
-rw-r--r--  1 apache  apache 19056 Oct 12 16:47 101D0-86_jpg_thumb_200x200.jpg

How can i give you a *short* but *complete* reproduce case?
The only thing i know is there are 2 files in the same directory, with the same permissions, that have 2 different behavoiur with the same function.
By the way, it doesn't matter my directory structure, cause I get the same error.
Anyway it's: '/var/www/vhosts/mydomain/httpdocs/"
I try:
<?php
	$tmp_img = imagecreatetruecolor($_GET['w'],$_GET['h']);
	$th_bg_color = imagecolorallocate($tmp_img, $r, $g, $b);
	imagefill($tmp_img, 0, 0, $th_bg_color);
	imagecolortransparent($tmp_img, $th_bg_color);
	$src = imagecreatefromjpeg($_GET['img']);
	imagecopyresampled($tmp_img, $src, 0, 0, 0, 0, $_GET['w'], $_GET['h'], $image[0], $image[1]);
?>

So they have totally different owners and groups.
Ok, at least this is a useful info.

Now please do the following:
<?
var_dump(file_exists("/var/www/vhosts/mydomain/httpdocs/images/products/thumbnails/101D0-86_jpg_thumb_75x75.jpg"));
?>
(or whatever the absolute path should be).

And tell me plz: safe_mode is On or Off?
And what's your error_reporting level along with display_errors ?

This is the message i obtain with your code:

bool(false)

I don't know how see your question about error, sorry...=((

Use phpinfo(); to see values of the options I was asking about.

error_reporting:
display_error=off
Local value=2039
Master value=2047

Turn display_errors On in the beginning of the code:
ini_set("display_errors", 1);
and see if there are any error messages.
Also, please tell me the value of "safe_mode" in phpinfo().

Where do i should see errors? in my first script?

Yes.
And I asked you about safe_mode.

Safe mode is on, and it seems there is no errors in my script. Simply the images aren't shown.

Read something about safe_mode.
No bug here - file_exists() and other filesystem funcs won't work with files that have different owner/group than script if safe_mode is On.

Ok, i got it.
Thanks a lot for your help.