In Dolphin 21.08.0, arch package maintainer removed kinit from dependencies.
Solution is to install kinit
pacman -S kinit
This bug is already reported https://bugs.archlinux.org/task/71840
Update:
In Dolphin 21.08.1-1 a patch is applied which allow terminal to open without kinit package
Dolphin was updated to 21.08.1-1 last night, and should be OK again