@Jos_Brands, the key detail here is that your Roon core is a Silent Angel product and is running an old version of RoonServer (build 911). You will need to install Roon 1.8 Legacy on this machine, but the process for doing so is in the purview of the people at Silent Angel.
Well in advance of launch we contacted all of the Roon Core partners to inform them of the requirement to provide a mechanism to install Roon 1.8 Legacy so Silent Angel should have a process for this. We recommend you contact them directly for information on how to move your core to 1.8 Legacy.