Since 2.23.0 there is something wrong with the z-index of the TelerikComboBox.
If a TelerikComboBox is used on a TelerikWindow, starting to type opens the dropdown behind the window and thus cannot be seen.
Clicking on the Dropdown Button of the ComboBox, the dropdown is opened and displayed correctly and after clicking on the Dropdown Button once, typing in the text field also displays the dropdown correctly.
It really bothers my users though that they cannot start typing right away so I had to revert to version 2.22.0 which does not have this error.
Note: Since I figured it's some kind of styling problem, I rebuild our custom theme using the ThemeBuilder but the error was still there after that.