I wanted to add custom buttons to WordPress editor so I installed Visual Editor Custom Buttons, made a new button (it’s really easy and intuitive) but when I wanted to create new post or page I encountered a problem: Visual Editor toolbar completely disappeared.
Long story short, plugin creates a new javascript file (button-1-1.js, …) every time you add a new button. The javascript file is created in the plugins js folder (it will most probably be wp-content/plugins/visual-editor-custom-buttons/js). The problem was that plugin somehow fails to write (insufficient permissions) to this folder so it is unable to create these files. After you created a new button you should open the aforementioned js folder and check if there are files named similar to button-1-1.js. If not then this could be the problem. You will most probably need to delete and add the button again after you change the permissions of folder js to 777.
The solution was to simply change folder permissions to 777 with FTP program (FileZilla : right click on folder and select File Permissions… option)