I have the same issue still in 2020. Both on windows and mac osx.
So we can better assist you, please provide a brief description of your current setup using this link as a guide.
Make sure to describe your network configuration/topology, including any networking hardware currently in use, so we can have a clear understanding of how your devices are communicating.
Mac Mini late 2012, most recent OSX, running roon core. Connected via ethernet to gigabit hub - huawei router.
iPad, intel nuc i7 (windows 10) and iPhone as remotes on wi-fi.
I was having this problem after updating my password on last.fm.
If you had already connected last.fm and Roon, you’ll need to go to https://www.last.fm/settings/applications to delete the old pairing.
After doing this, I was able to update my last.fm account in Roon with the new password.