The upload problem is still there even after HMWP plugin is disabled. So it seems like the issue is not related to HMWP plugin. However you can try these:
– change permissions on my /wp-content/uploads folder to 755, and then to 777 just to see if it helps
– re-upload core WP files (/wp-admin, /wp-includes)
– replace .htaccess file content with default WP htaccess – https://codex.wordpress.org/htaccess (Basic WP)