What version of ACF are you using?
Is this with a front end form or in the admin?
Have you tried uploading images for the field?
turn on WP error reporting and see what kind of errors you get. Add to wp-config.php:
define('WP_DEBUG', false);
define('WP_DEBUG_DISPLAY', false);
The two errors you’re listing are after the upload so there is some reason that the file is not being uploaded. for example, PHP has not save the uploaded file to the temp folder.
Do you have a limit to the number of files or amount of space you can use on the host? There are a whole host of reason that could prevent a file from being uploaded.
