Thank you for your investigation. I don't know who or what did call recode.php. I will have a look at the serverlog.
Is there an option to mark this tread as solved?

-- Update --
Thanks to @sonichouse

Some times I'm that stupid!!!

I wanted to find out where the problem is. If only developer use this file, why is it in the offical source?
But if I don't call recode.php and do nothing on my blog instead of viewing it, then from time to time the UTF-8 files are all overwritten. I can simulate this by requesting recode.php.

I have a problem on my blog that sometimes the values in the "lang/UTF-8/serendipity_lang_de.inc.php" has been overwritten by s9y and all the special characters and mutated vowel aren't UTF-8 at all. At the moment I disabled (chmod u-w) the write access on this files.
Is there an error on ...