Created attachment 114429 [details] Configuration dialog, showing the error message. I am unable to get new splash themes on Debian stable, due to the error mentioned in the summary. I looked into /etc/xdg and indeed there is no ksplash.knsrc file in there. I tried reinstalling plasma-desktop and kde-plasma, but that did not help.
*I am using Debian testing, sorry, not stable.
Fixed with https://commits.kde.org/plasma-desktop/465e86ee7b947dd3a4d8655b00640e21760dd90c