Can't play in Realms, Internal Exception: java.io.IOException

Everytime I try to join my friend's Realm, I get kicked out in 3 seconds and there's a connection lost message "Internal Exception: java.io.IOException Etäpalvelin sulki aiemmin luodun yhteyden" (the end is in finnish, roughly translated as: An existing connection was forcibly closed by the remote host).

I have tried reinstalling java and minecraft, changing internet connections, disabling firewall and antivirus, restarting router and computer etc.