When trying to open an http(s) link in Dolphin, the webbrowser should be launched instead.
The codepath didn't take into account that it can be either a service or (when prefixed by an exclamation mark) an exec path. This had it try to exec the name of a desktop file which obviously didn't work.
The fallback to Konqueror is removed as its file viewer is just a Dolphin Part and it isn't installed by default these days anymore.

Set Browser to chromium.desktop, entered http URL, now it worked, before it said "command chromium.desktop was not found".
Set Browser to some console command, entered http URL, console command was executed
Didn't find a protocol that Dolphin couldn't handle but wasn't http (where the previous Konqueror fallback would kick in, gopher:/ perhaps? :)

Browsing file systems, zip folders, camera:/ still works