End portal crashs server

Every time me or any friends try to enter the end via the portal on the realm it crashes the server disconnects us all for about 10 minutes this seems to happen in realm and in normal version of the world any help or advice as its a old world but we've done slot of work since it was transferred over and I don't want to loose it all

My game states version 1.2.1 but does not allow me to select that on the versions list