Steps to reproduce the issue

1. Create some articles if you haven't already.
2. Open the backend dashboard.
3. Edit the (backend dashboard) module "Recently Added Articles".
4. Set the order to "Recently Modified First".
5. Refresh the backend dashboard.

Expected result

Articles should be displayed in order of the modified date.

Actual result

The backend dashboard displays:

Error 500 Unknown column 'modified DESC, created' in 'order clause'

System information (as much as possible)

PHP 7.2
Joomla! 4.0-dev as of today, also replicated by other users in beta-3.

Additional comments

Issue was originally reported by user winterman at the German support forum: https://forum.joomla.de/thread/12211-error-500-in-4-0-0-beta3/