Please try using this CVS snapshot:

  http://snaps.php.net/php5.1-latest.tar.gz
 
For Windows:
 
  http://snaps.php.net/win32/php5.1-win32-latest.zip

If you're still able to reproduce it - please put the code somewhere in the net and paste the link here.

I can't install this php version at work, so i can't answer.

I just make a precision. This bug append when u write in a only line :
echo htmlentities(html_entity_decode("Σερβία και
Μαυροβούνιο"
, ENT_QUOTES, "UTF-8"), ENT_QUOTES, "UTF-8");

Add a space between α and κ
and between ι and Μ

>I can't install this php version at work
You don't to install it, you just need to compile and test it.

Please upload the test case as I asked.

Remove the 'utf-8' from those funcs and it works fine. 

I failed to install php5.1-win32-latest.zip. I've "Unable to initialize module". I installed wamp server at home with php5.0.1 and htmlentities works.
I can't remove UTF-8 because greek strings come from database (html_entity_decode is just to reproduce errors)

so, i patched my function
if ($greek == "Serbia kai maurobounio") return $greek;
else return htmlentities($greek, ENT_QUOTES, "UTF-8");