Plymouth seems to be running, but also terminating within a couple of seconds. Since it's designed to not display anything if it thinks it won't run for very long then I think failing to see a Plymouth boot logo is OK in this case. You might like to try the workaround for bug 1869655 which is to add kernel parameter:
plymouth.use-simpledrm I don't know much about sddm, Plasma or KDE, but there's a clear difference in the logs. When the screen remains black you're also missing the sddm log message: Display server started. So it looks like an sddm bug sometimes failing to launch X. Have you installed a graphics driver manually, or are you using the Ubuntu default drivers? ** Package changed: linux (Ubuntu) => sddm (Ubuntu) -- You received this bug notification because you are a member of Kubuntu Bugs, which is subscribed to sddm in Ubuntu. https://bugs.launchpad.net/bugs/2063143 Title: Frequent boot to black display To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/sddm/+bug/2063143/+subscriptions -- kubuntu-bugs mailing list kubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/kubuntu-bugs
