I managed to fixe the issue using the advise below: I copy pasted the answer I got from another member:
You have got an incomplete installation. Must be an installer issue. Can you remember what installers and in what order you installed? I'll work together with Slitherine guys to find out what went wrong here.
In the mean time, you can download the correct content of this folder here:
https://www.dropbox.com/s/7e4crv4yga8a3 ... 9.zip?dl=0
If you copy the content of this archive into the folder you've just shown on the screenshot, the issue should be gone.