It might have been because I was running Roon and RoonServer on the same machine (although that worked on 1.8), but in any case I resolved the issue by first migrating the core from the old Mac mini to the new Mac mini sticking with the 1.8 version. After 1.8 was running on the new Mac mini, I installed RoonServer 2.0 and connected with Roon client from a different machine. Seems to be working now.