I had the same, but my Core is Linux on Debian. A reboot of the Core NUC fixed it. Did you reboot your Core PC after the update? Reboot shouldn’t be necessary after an update, but was needed this time - on Debian Linux anyway…
My core is on Ubuntu 18.04.4 LTS. I updated to build 555 without any problem, reboot unnecessary. So, the problem you’ve experienced is not the same for all.
Since you are using a Windows PC, I would verify that both Roon.exe and RAATServer.exe have been added as exceptions to your Windows firewall.
You can use these instructions to add the exceptions and the executables themselves would be located in your Database Location/Application folder path.
I would also add these exceptions to any Antivirus or other Firewall blocking applications you may have and ensure that you connected to your network via a Private network, not a Public one, see this guide for more information.
I have sometimes seen Windows remove Roon as an added exception, so I would check this aspect and re-add the exceptions (even if they’re already there) to see if it helps.