Upgrade issues with Manager

A client used the upgrade option from the dashboard to upgrade to 3.x.
The site is mostly working on the frontend, with a few issues. However the backend is inaccessible.
The error thrown in the apache error log is:

PHP Fatal error: Uncaught Error: Class “modManagerRequest” not found in /…/core/src/Revolution/modX.php:1518
Stack trace:
#0 /…/manager/index.php(56): MODX\Revolution\modX->getRequest()
#1 {main}
thrown in /…/core/src/Revolution/modX.php on line 1518

Unfortunately the client did not have backups enabled at their hosting environment, so I can’t just revert with a backup.

Any help would be greatly appreciated.

Is it only the start page of the manager (manager/) that doesn’t work or all pages of the manager (e.g. manager/?a=system/settings)?

If the problem is only the manager start page, then there’s maybe a widget that is not compatible with MODX 3. Try removing the widgets from the dashboard (database table modx_dashboard_widget_placement).

If the same issue occurs on every manager page, try (temporarily) deactivating plugins that runs on manager events (database table modx_site_plugins → column disabled. Clear the cache folder core/cache/ after making changes to the database table).

Also try (temporarily) deleting the value of the system setting “extension_packages” (database table modx_system_settings) to see if that fixes the issue.