Bug/Issue Unable to opt into Beta with Steam

I'm not sure if this is a Steam issue or an issue with Stardew Valley, but I'm unable to opt into the multiplayer beta. Whenever I try to enter the password (which I'm certain I'm putting in correctly), the window simply closes and upon returning there is no option in the drop-down box.

The strangest thing is, it used to be working fine. I'm unfortunately unable to offer exactly when it stopped working since I took a break from the game for my exams lol.

Would appreciate any help, and if this is a Steam issue then I apologize for taking your time.