Hello @Ralf_G ,
That error message looks like the Roon Server isn’t able to use the proper certificates. Please note that Roon 1.8 is no longer under active development, any sort of resolution would come in the form of configuration changes on the affected Mac.
If you are unable to update to Roon 2.0 due to the Mac being too old, you may want to check out a tool that a few of our users suggested in the past (but please be warned that we have no affiliation with this tool and cannot provide any specific guidance):