Our platform is built somewhat different than a normal hosting stack is made. This means that also the file permissions are somewhat complex. It can be possible that you can't create or edit files on SFTP after for example a WordPress or plugin update.
This problem can easily be fixed with a click on one simple button. Go to the control panel, then to the environment where you have the permission denied error. Then go to the 'Settings' page and then click 'Reset file permissions'.
After clicking reset it can take up to a minute before all permissions are reset. This action will not change or delete any data.