We have been using realms for months now and haven't had this issue. I am the owner of the realm and I can log in, but my kids suddenly can't. It's been happening for a week or so. They get the error. 'Unable to connect to realm; Failed attempting to join realm. Please try again later.' They are successfully connected to Xbox live, if I log into the same device as the owner I can get on the realm, but as soon as I log the kids friend accounts back in it does not work and get the same error. We have reset tablets, deleted minecraft and re-installed, tried different networks.

I also have a second realm and when I tried inviting one of them to the realm, they got the invite but when they press the check mark nothing happens.... only the decline button works.