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?
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?