No worries… 403 permission error means that a file or directory (folder) cannot be accessed by the person because it is protected and that they do not have permission to access it. This can be a result of scripts in a plugin or even on the server (host) side. Some web hosts automatically put restrictions on some kinds of scripts and access.
For yours, it was strange because it was random for me and it gave me the Access Denied error message then later it didn’t.
As usual, if anyone has a problem, troubleshooting it means to take note of when something happens and if it happened before plus what changes were made before the error or problem shows up. Plus, disabling plugins and then one by one turning them on and testing the site each time will determine if a plugin is causing conflicts with the site. So for yours, you could try that and see if the menu doesn’t disappear.
This is just part of making websites…it’s building, testing, troubleshooting, fixing, adjusting, more testing….