This may not be the only cause but I have found that the reason I am getting this error is because when the player tries to join the realm, it asks to download a resource pack (in this case I had added a new one after the update to remove the result of MCPE-105882 by replacing the texture with a blank one), however shortly after this the game decides the popup box is actually a wifi issue and shows the 'unable to connect to world' screen.
A temporary fix in this case would be to remove any resource packs from the realm OR spam the button you would usually use to press ok whilst joining (on PC have the cursor where the download and join button would appear)
This may not be the only cause but I have found that the reason I am getting this error is because when the player tries to join the realm, it asks to download a resource pack (in this case I had added a new one after the update to remove the result of MCPE-105882 by replacing the texture with a blank one), however shortly after this the game decides the popup box is actually a wifi issue and shows the 'unable to connect to world' screen.
A temporary fix in this case would be to remove any resource packs from the realm OR spam the button you would usually use to press ok whilst joining (on PC have the cursor where the download and join button would appear)