That's caused by a incompatible 3rd party extension.

Well, back to the sleuthing, then! :-(

Back end template is Isis, so isn't that CORE? Or you are saying that the main window of backend is being over-ridden somehow by a 3rd-party extension?

All of our extensions were 100% compatible with Joomla 3.7.5 - I really don't expect a minor release to break stuff... Just though I would pass this along since I am sure Core developers want to be fully aware of how their changes interact with the larger eco-system!

Are you using SP Page Builder? If yes, check out this #18034.

Not using SP pagebuilder...
3rd party extensions are
Community Builder/CBsubs
Kunena Forum
K2
( all updated, and all those devs swear they are 3.8-compatible... )

Also see this: https://docs.joomla.org/J3.x:Call_to_undefined_method_JAdminCssMenu::addChild_in_backend

Thanks! "Changes in the administrator menu module to support new features." - where can I find the exact places where said changes were made?
Is it this change?

220802a

I don't see how that's relevant, but I don't see anything more recent in there...

Please consider posting to the Migrating and Upgrading to Joomla! 3.x forum for further assistance.

Edit configuration.php
change
public $debug = '0';
to
public $debug = '1';

change
public $error_reporting = 'default';
to
public $error_reporting = 'maximum';

Do you now get the file at where the error occurs ?

Ah-ha! yes it's something that was in an extension.
Call to undefined method JAdminCssMenu::addChild() .../administrator/modules/mod_jomclassifiedsmenu/tmpl/default_enabled.php:15

$menu->addChild(new JMenuNode(JText::_('JOMCLASSIFIEDS'), '#'), true);

JMenuNode has been deprecated? (sigh) I guess I have some digging to do...

@rotech1 Since this is a 3rd party extension issue, this can be closed.

OK! I found THIS, so I'm on my way to hacking up a fix!

https://fossies.org/diffs/Joomla/3.7.5-Stable-Full_Package_vs_3.8.0-Stable-Full_Package/administrator/modules/mod_menu/menu.php-diff.html

@rotech1 This is simple filed update. Open a topic as suggested and will assist.

@wojsmol

since we have already assisted @rotech1 above to find the issue
you mean open it in Joomla forums ? (not here , right ?)

People are asked to update 3rd party extensions,
but in some cases do not update non-free extensions ... (some of the issues opened here lately were because of not updating non-free extensions because the subscription has expired)

Also @rotech1 , why are you tryng to update the extension manually ?
better try to update the 3rd party (non-free) extension to its "latest" version (if an updated version is available)

Yes as requested in #18152 (comment). You're right that it's better to update paid extensions from official sources and not modify them.
BTW Do you remember the issue number of the about first level categories not displayed?

I am using a customized version of the extension, so am obligated to maintain it manually. The maker hasn't released their 3.8-fixed version yet, but when they do ( they promise to in a couple of days!) I will look at their changes.