Forum admin > Components > Editor (TinyMCE). Thus is where you set which TinyMCE buttons and TinyMCE plugins you want to use.
The TinyMCE plugin ‘wltags’ had been added to the plugin list. This explains why it was trying to load it and it was reporting a failure. It does not work with the use of the TinyMCE editor on the public side of your site – only in the Admin.
So I simply removed it and set the options back to their default state. By the way – all being well we have a fix for the heavy font being seen on the buttons which is a result of changes to WordPress in 3.8. We will be releasing a 5.4.0 Simple:Press either today or tomorrow.