This most likely means the server you're trying to connect to isn't 100% online. Whether it's crashing after starting up or something similar, you should try looking into plugins like skript that could easily cause the server to crash.
If you spawned in too many entities, you may want to run `/kill @e[type=!player]` in console and then try connecting.
Other than that, if the issue still persists, I would recommend making a support ticket.