I use Dolphin 18.04-2, Plasma 5.12.5, Frameworks 5.46 and Qt 5.11
In Dolphin I always select Preview mode to see pictures, video-stills, texts instead of file icons. This does not work anymore.
Also the preview in the F11 preview panel doesn't work, but that is probably one error.

What I also enabled is: Control ==> Additional information ==> Size. It shows the files sizes and for folders the amount of items (files and folders) which are in that folder.

A third thing I noticed, when I open a folder which contains several other folders, the size information is shown very slow. It can take 2-3 seconds per folder to see how many items there are in that folder. When I switch off preview the info is shown immediately.

When I now click a text file, Kate is no longer opened but I see an error: KDEInit could not launch '/bin/kate'.
When I click a png, a jpg, an mp4, a pdf file I see no error but the corresponding program to open that type is not started. Nothing happens.

I downgraded Dolphin up to 16.12 but that didn't make any difference. I know for sure these older versions always worked fine, now they don't, meaning something else is causing these problems.

Does anyone here also have these issues, or maybe found a solution to it, or knows where to start looking?
Please tell. Thank you.

Have you logged out and back in or restarted since upgrading (or downgrading)?
If the problem persists after restarting, does the problem come up when starting plasma with a new user?

I got the tip to start Dolphin with "dbus-launch dolphin" and that helps. Now it all works.
Since the latest KDE updates it also works without the "dbus-launch". No idea what caused it but it works again.
Thank you for your answer.