Hello there!
I recently encountered a sudden issue. The "@ message" shortcut no longer works if I attempt to follow up with another PM to the last person I started to send a private message to. Instead, I get this error: "Unknown or offline player: null." However, things work fine if the PM I'm sending with this shortcut is a reply and not a follow-up. For example, if someone sent me a PM saying "Hello Muhammad," then I proceeded to type "@ Hey there buddy" in the chat box. I don't know how long ago this bug appeared, because I haven't logged into the playroom since two days.
Edit: Oops, I didn't know that it's a new feature to press double at @@ to send to the last person I sent to, and a single at sign for replies as we used to. My apologies.

Strange bug. hoping it to be fixed soon.

The person already explained in their post that they weren't aware of the new feature, so there's nothing to be fixed.