Hey,
I have fixed the Problem. The variable $rcmail_config['mime_types'] was the problem. I have downloaded a mine.types file and saved it in the config folder. After that I changed the variable to $rcmail_config['mime_types'] = INSTALL_PATH.'/config/mime.types';.
Now is no new error in the error log.
The error has since switching to PHP 5.5. Has any other this Problem with PHP 5.5?
Or maybe what was changed in PHP 5.5 on mime.types?
Thanks for your help!