Do you have the correct php version, correct extensions enabled, you have to check all that stuff.

I think so, yes. Does Laravel 5.6 need anything more than 5.5 did? Because I can roll back to 5.5 and get it running.

https://laravel.com/docs/5.6#server-requirements

Yes, it meets all requirements as far as I can see. The only "new" requirements are:

XML PHP Extension
Ctype PHP Extension
JSON PHP Extension

And those are default extension in windows binaries. I also double checked in phpinfo()

Hang on, you got me on something there. I saw error_reporting was disabled in php.ini

After enabling it, I get more ouput:

Fatal error: Uncaught Error: Call to undefined method Illuminate\Foundation\Application::configureMonologUsing()

I am now investigating on that

Try my above suggestion, do a new install and migrate models, views, etc over to the new install. I have no idea why you can't upgrade. I even went from a 5.5 to a 5.8 on one of my installs.

I do not go by the "upgrade" guide, I use winmerge and completely update the app (laravel) first, then and only then do I do a composer update. Of course I check new server requirements.

I am guessing you cleared composer cache and things like that in between.

Also see https://laracasts.com/discuss/channels/laravel/error-templates-not-updating-56-57-57?page=1

I am just trying that now..

"I am guessing you cleared composer cache and things like that in between."

Sure I do!

Double check that php version also.

PHP Version is 7.3.11.

I am still stuck at this:

Fatal error: Uncaught Error: Call to undefined method Illuminate\Foundation\Application::configureMonologUsing() in \bootstrap\app.php:54

Have you checked bootstrap\app.php:54

is there something you missed in the upgrade guide.

Is that error on a new install or are you trying to upgrade again.

You have got to have something somewhere in a config that references a package that the installer does not like, in other words something somewhere needs to be commented out for this to work.

But honest I have never had this trouble, and I have done many upgrades.

I got this error when I tried to upgrade. A fresh install worked and migrating all the components showed that JWT package needs some work. I got this working and now I seem to have trouble with the throttle middleware, but I am getting there.

Even though I think it is bad to have to go that way, it helped me finding a way to fix this issue.

Thank you!