There is no know exact cause for this. However, it seems to usually be caused by a bad connection between the server and the client. This can be caused by a flaky router (turn off your router and turn it back on), a flaky server (shutdown Minecraft and restart your computer), bad connections (unplug your cables from your server and router and plug back in. Same with client.). Bad cables (try replacing your Ethernet cables). Bad wireless connection (try dslreports.com/speedtest to test). Different versions of java between client and server (so I've heard).

On both the client and the server go to http://dslreports.com/speedtest or https://secure.dslreports.com/speedtest and test your speeds. They should be the same. If one is slower than the other, try to find the connection issue and fix it.

Other solutions include re downloading Minecraft as it may have gotten corrupted somehow with the mods. I would try both client and server versions.

Good luck.

So, I am currently making my server on minecraft and now when I join I get a message that says "Internal Exception: java.io.IOException: Connection reset by peer". I can successfully join the server, but I get this message every 1 second-10 minutes. Everyone else does not have this error. So, how can I fix this? Thank you, in advance!