What worked for me was accessing the 'multiplayer' tab and selecting a server, joining, then cancelling before I connected. After this I could access the realms tab without the message popping up. I did not get the red text error and it allowed me to join the realm. The red text was usually solved by restarting the game.
This is what worked for me, the other solutions shown here did not, and I ended up just playing about with what I had to see if anything worked and this ended up doing that. It is possible that all you have to do is wait and this was coincidental timing, although I had only been trying to solve it for about 5-10 mins.
Wren
What worked for me was accessing the 'multiplayer' tab and selecting a server, joining, then cancelling before I connected. After this I could access the realms tab without the message popping up. I did not get the red text error and it allowed me to join the realm. The red text was usually solved by restarting the game.
This is what worked for me, the other solutions shown here did not, and I ended up just playing about with what I had to see if anything worked and this ended up doing that. It is possible that all you have to do is wait and this was coincidental timing, although I had only been trying to solve it for about 5-10 mins.