java.io.IOException: Connection reset by peer

Basically a Connection reset by peer indicates that the other side (in this case the player) did unexpectedly close the connection. This can have a lot of reasons: Either the player has an unstable connection (especially when using WLAN, for example), or he just cancels the loading procedure by pressing ESC, or his game crashes, or he just left the server. Unfortunately Connection reset by peer doesn't provide any further information about what's going on exactly, it's rather a generic error

If some players cannot join a certain server (while they're able to join other servers without issues), it's possible that there is too much data the server is sending to the client. This can either be lots of custom images or too many buildings in the spawn area (for example, if the spawn building consists of hundreds of thousands of building parts, this can be a major problem for players with lower hardware), or plugins downloading too many assets like custom models (loading process halts until all assets were downloaded).

If it's an old world (created before the npc update last year), it's also possible that there are too many npcs in the world. In old versions there was a chance that npcs fell through the ground, resulting in many npcs being lost somewhere in the underground (never being seen by a player, but still having an impact on performance). Some servers had issues in the past if there were more than 100,000 npcs in the world. In this case it helps to delete the npcs (the deleteallnpcs command removes all npcs in the world, optionally you can restrict it to a certain npc type and a radius).

Oh, I'm glad to hear it works now