The server should be able to be joined.
And regarding to the issue, I can't do anything about it, I'm so sorry. I really looked at the configuration file (paper-world-defaults.yml) and can't find anything related to the issue. If you found something from the internet related to this issue, please let me know on the #privatesmp-chat and give me your full explanation how to fix it. Once again, I'm so sorry for the inconvenience and I really can't do anything about it.
I did some changes on the server.properties file, the network-compression-threshold was set on 512 and now, I set it to 256.