Please, try to turn off the nice scroll option located in the Customize-> General-> Extra menu.
and check again.
If the nice scroll is turned off and the problem still exists, then maybe you have a conflict with some of the activated extension. To verify this, try to deactivate any additional extensions and check again.
If the above solutions didn’t help you, please give me your WP login credentials in the private reply (check the “set as private reply” checkbox below the post) so I can check the issue directly on your site.