Can't add items to iTunes library

Starting this morning, every time I try to add an item to my iTunes library, a blank window appears. I can close it, and in the main iTunes window there's an exclamation mark in the top right corner. If I click on it, a dropdown appears with the information: Adding item to library cancelled.

For clarity: I am not trying to add new files, I'm just browsing the iTunes catalog and trying to add items from there.

I have an active Apple Music Account so I see no reason why I should not be allowed to add an item from the iTunes Store.

Any suggestions on what the problem might be and how to solve it? Thanks.

MacOS Mojave 10.14.4 / iTunes 12.9.4.94

It's a known bug since the release of macOS 10.14.5.

The blank window that shows up when you try to add an item to your library is actually supposed to contain new terms of service that you have to agree to, but it's bugged on macOS and just displays an empty page instead.

The only solution right now seems to be to go to Apple Music on any of your iOS devices (iPhone, iPad) and add a random item to your library there, then restart your iTunes on Mac and it should be fixed.

This bug has been around since 10.14.5 Beta and Apple still hasn't fixed it, which blows my mind, because it basically makes Apple Music unusable for anyone without an iOS device.