Steps to reproduce the issue

• Run SQL command: UPDATE #__menus SET checked_out=0 WHERE client_id=0
• Click on System > Maintenance > Global Check-in

Expected result

You should be able to check-in menu items.

Actual result

There are no tables with checked out items or there are no tables with checked out items that match your search.

You can restore your DB by: UPDATE #__menus SET checked_out=DEFAULT WHERE client_id=0

System information (as much as possible)

Joomla 4.0.0-beta4

Additional comments

If any of the tables, say #__menu, contains items saved with older Joomla 3.9 component, they may have items with check_out being 0. Unfortunately, a few Joomla models only check against user_id and NULL, meaning that the items may/will be semi-locked forever with no way to update them.

administrator/components/com_checkin/src/Model/CheckinModel.php should be changed to check against something else than >=0. In fact, I don't think that WHERE clause is needed at all.