Hi @Matti_Malkamaki,
I know you mentioned that you previously took a look at the firewall, but in case you need the instructions again:
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I would verify that both Roon.exe and RAATServer.exe have been added as exceptions to your Windows firewall.
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You can use these instructions to add the exceptions and the executables themselves would be located in your Database Location/Application folder path.
Are you running any other kind of antivirus of firewall on the PC? Even AV apps such as McAFee/Kaspersky/Avast/ect. can sometimes interfere with Roon communication, so I would temporarily try disabling these as a test.
If you have checked this aspect, then my next suggestion would be taking a look at the Audio Drivers as @Rugby suggested above.