Editor scene view: Unable to click on UI gameobjects (2018.3 +.4)

My project started to exhibit the problem, that I cannot click on UI elements (Canvas, Text, Dropdown ..) in the editor scene view anymore.

When I click on geometry, e.g. a cube, it goes as expected, and the gameobject is selected in the hierarchy window and the gizmo appears. The same happens as expected, if I select the same UI gameobjects in the hierarchy windows.

Does anyone have an idea what I could have misconfigured? Or is it a bug ? The project went through Unity upgrades since Unity 5.x time.