Weirdly the menu buttons disappeared.
Kdenlive 18.04.2 flatpak or Kdenlive 18.04.1 appimage. Both experience the same issue with the menu. Not the old Kdenlive 4:17.12.3-0ubuntu1 though. What could it be?
I have to mention that I reinstalled them multiple times without any luck - also deleted the config files.
Make sure you delete ALL Kdenlive files (search Home folder for them). Uninstall Kdenlvie, then delete all files, then everything works.
Did you changed your theme? Basic buttons are referred to the icon theme you use. If so change to the default one test if this is the reason.
I'm searching about this issue, and found this post solving my problem
I think KDEnlive is still using the KDE shortcut environment.
So Ctrl+M absolutely solved the problem.
Go to Settings - Configure Toolbars
in Avaliable actions: type Render
Choose it and klik right arrow button