Frustration is a big part of coding. I'd recommend wine, beer or something stronger, as required.
Any error/exception being thrown will break the toolbar if it stops script execution.
Your problem has to do with the paths that have (or have not) been correctly set somewhere, either in the CI framework, or in your server set-up (in one of the config files)... it may even be a problem in one of the .htaccess files (though that is probably an outside chance).
Also, whatever webserver you're on will have a lot of header-setting options that can be applied outside of the CI framework to tighten or relax security).
I'd suggest starting to look in App.php - there is a $baseURL variable spec'd there... and to get it working, perhaps checking $CSPEnabled=false is set (which I think is probably the default) - the idea is you want to have those two paths (in your cut&paste) the same (first prize), OR the CORS policy to be relaxed (as a temporary measure so you can work with the code).
Also, if need be, if you're using a local server (which I'm guessing you are, based on some of the cut&paste above), you can play with a remapping in your operating system's HOSTS file if necessary.
