EDIT: upon further testing, I couldn’t confirm the glitch below. Rather it seems that I had put Is Enabled to false for some parent widgets. And IsEnabled = false disables all child widgets quite strictly, it seems.
ORIGINAL POST:
I found the error and I blaim a glitch in the Unreal Engine.
Following advice from my girlfriend, I moved buttons around in my UMG Widget hierarchy that looks like this:
Finally, the solution to the deactivated buttons:
Once I replace the retainer box by deleting it and putting it there, again my buttons (both, UButton and MyCommonButton) unfroze from their forcefully disabled state.
In theory, there could be a setting of the retainer box that I basically reset by deleting it. But I can’t find a setting that reproduces the forcefully disabling of buttons inside it.
This is why I think it’s a glitch.