I hope this works for everybody else who's looking for this topic.
open really-simple-captcha/really-simple-captcha.php
214 foreach ( $suffixes as $suffix ) {
215 $filename = sanitize_file_name( $prefix . $suffix );
216 $file = trailingslashit( $this->tmp_dir ) . $filename;
217 if ( is_file( $file ) )
218 unlink( $file );
you have to replace "trailingslashit" to "untrailingslashit" on line 216.