Unable to upload multiple files using the WordPress Media uploader. Cause of the problem: ModSecurity. If ModSecurity is set to SecRuleEngine DetectOnly in the modsecurity.conf file then it will still randomly break file uploads due to a problem with ModSecurity not being able to open some temporary files for reading. Solution: change the modsecurity.conf setting to: SecRuleEngine Off.
Related topic: https://forum.ait-pro.com/forums/topic/file-upload-does-not-work-no-errors-modsecurity/