Hi,
I installed from the repository, but when I try to start I the following error message appears for a few seconds and thats it.
Any idea how to fix this?
Moinchen & welcome, @Horst!
What operating system are you using?
32-bit XP?
MfG
Claes in Lund, Schweden
Sorry, I forgot to mention: I am running Kubuntu 16.04.4 LTS
Hm. 16.04.4 is a bit old 
When you say that you installed from the repository,
what steps did you take?
See link RawTherapee - Downloads and select
Ubuntu / 64||32 bit / All types in the dropdowns.
The easiest way is to follow the instructions given under
sub-heading Repository 1 - PPA
Here is a longer thread about RT and your distro:
MfG
Claes in Lund, Schweden
In addition to what @Claes wrote,
http://rawpedia.rawtherapee.com/How_to_write_useful_bug_reports
http://rawpedia.rawtherapee.com/How_to_fix_crashes_on_startup
I used this guide from http://rawtherapee.com/downloads/:
Repository 1 - PPA
This repository is kindly run by Dariusz Duma, and all questions should be directed to him.
To install RawTherapee follow these simple steps:
Open the terminal and paste this code into it:
sudo add-apt-repository ppa:dhor/myway
sudo apt-get update
sudo apt-get install rawtherapee
If you want a development version:
sudo apt-get install rawtherapee-unstable
RawTherapee should now be installed and ready for use.
EDIT: Just installed the unstable version for testing and its working.
Moin, Horst!
Good! Enjoy it
MfG
Claes in Lund, Sweden