As of the recent hotfix, I'm unable to play online multiplayer, including servers or friends' worlds. The issue also extends to my purchases from the in-game market store, which does not recognise any purchases to my account, when I in fact own multiple products from the marketplace. Where servers or multiplayer worlds should appear, there is instead a message which roughly states "You can't play online because of how your Microsoft account is set up. You can visit (link to Microsoft account settings website) to change your account settings." I did this and double-checked that my settings allowed for multiplayer, which they do, and the issue does not affect other games which require a Microsoft account to play online. Therefore the issue is solely affecting Minecraft. I have also double-checked I still have my Xbox Live Gold subscription, which I do. 

So, basically, it seems to be a bug with Minecraft.