The screen used to connect to realms will just fail to load, staying at a blank screen with only the "back" button clickable. Sometimes, the realms I'm in will appear, sometimes I have to go back and return to the Realms screen again, and sometimes I'll get an Error 500 message.
This isn't a problem with my internet. Absolutely every other web-related service on my PC works just fine. I've tried this both with default Minecraft and the Optifine version with the same problem. It feels like whether or not I'll be able to play on my Realm is a coinflip at this point.