I debugged this issue, and it’s turn out that the temp-files doesn’t have the correct rights for removal.
On line 69 till 73 in wp-content/plugins/really-simple-captcha.really-simple-captcha.php the temperary files are set to closed permissions, so they can’t be deleted afterwards. So I changed the permision to:
/* Mode of temporary image files */
$this->file_mode = 0777;
/* Mode of temporary answer text files */
$this->answer_file_mode = 0770;
