I observed this behavior in the past and now again when the update from 1.2.9 to 1.2.10 was done. The Minecraft Client on Windows10 updates automatically to 1.2.10 and the Realm still has version 1.2.9. Therefore you are not able to play anymore on the realm
The realm only updates, when the OWNER of the realm logs on to it with version 1.2.10. Invited friends can not play anymore until the owner logs in again.