During the load process (Breeze start screen) I get a notification for some milliseconds. Something with Kontact, but I cannot read the text. Kontact is started automatically and runs well. So probably it says all works well.

What kind of notification? A Plasma style notification?

Looks like old school information. Center top, themed, small frame around. Maybe I'll find a high speed cam to capture it.
BTW: Quite ugly is the jumping app cursor during the boot process which is related to this issue, I guess.

Are you logging into a saved session?

No, its just the normal boot process.

https://www.youtube.com/watch?v=B-KBGJ0XQg4

It's the first time that I get this low battery notification on start-up. I think we should improve all those notifications. My problem is the short message at ~20s. I still cannot read anything

Heiko Tietze wrote:It's the first time that I get this low battery notification on start-up.

Yeah problem is that kded starts up which launches PowerDevil which checks your batteries and then immediately invokes said notification before Plasma's notification applet is ready which makes it fallback to the old notification style. Fixing that has been on my todo list for ages. :/

So at least the reason of this unsightly popup is clear.

And the "unsightly popup" has just been fixed in current master (5.2 will have the fix).