When a realm is opened, it gets disconnected after a few seconds and reads the error "Internal Exception: java.IOException: An existing connection was forcibly closed by the remote host". After a few times of restarting the server, it stops happening. It happens too frequently to be looked over.
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This appears to be a duplicate of REALMS-3593, which hasn't yet been resolved. If you have any additional information to provide regarding this issue, feel free to comment on the parent ticket. If you feel that this issue is affecting you, feel free to vote for the duplicated ticket, to give Mojang a basic idea of how many people are impacted by the problem. Just to further inform you, this will not determine how quickly the bug gets handled or fixed.