Hi Marius,
To begin with, I have to admit that I am no expert when it comes to Linux related topics. However, looking around quite a bit, I was able to determine the issue (or at least find a solution 
).
The issue (at least I assume so), seems to be a missing correct link (symbolic link or symlink) to Java, which causes OpenHAB not to find the Java installation correctly.
I am quite certain that there is a SSH-solution to solve this (and anyone with slightly more Linux experience will likely find this to be the more obvious solution), but I preferred a very simple way:
- Uninstall Java from your Synology.
- Open Synology Package Manager and add the following package source (Package Manager → Settings → Package Sources → Add): https://get.filebot.net/syno/
- From the “Community” part of the Package Manager, install “Java 8 Installer” from the author rednoah.
During the installation process, it will download and correctly install the latest Java version.
Afterwards, I was able to install OpenHAB without any issues. Let me know if it worked for you as well.
Cheers,
Joseph