To be honest, I also faced this issue yesterday. My first thought was, "Am I banned?" I immediately joined someone else's room. Then another thought raised, "If I am banned to create rooms, I should be banned to join as well.” This led me to investigate this case further. I quickly clicked on drrr.chat and searched if people faced this before or not. And AHA! They did, so I was sure I wasn't banned at that point.
I'm not sure why it occurred. Maybe, someone created many rooms at once, and it reached the limit of rooms, so new room option was no longer available.