Comment # 12 on bug 1184817 from Fabian Vogt

You can press escape when the splash screen shows up to see more info. Or just
uninstall plymouth (and regenerate the initrd) to get rid of it, what I
recommend anyway.

I don't think this is related to Plasma, it might be some issue with the
storage layers. Reassigning to kernel, hopefully they have an idea how to get
more info about this issue.