Simple Captcha is a plugin used to add captcha in contact form 7 but after updates it is not working
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;