Today I spent some time investigating what happened. I recall clicking on something too quickly while the dialogue box was opening and it went crazy from then onwards.
Rebooting didn’t fix it either.
However Process Monitor from Sysinternals allowed me to locate the cause of the corrupt layout and missing files pane:
Rename or delete the current user\software\microsoft\windows\currentversion\explorer\
cidsave\modules\globalsettings\sizer\commonitemdialogsizer key value