Problem with the minecraft server "Existing connection has been abruptly sh

I host a minecraft server on my computer (via a router, without hamachi), my friends connect via my external ip address, some connect without any problems, but there are two that often crashes during the connection, as if they enter the server but after a few seconds the chat appears on the chat information about their exit and they get the error "The existing connection has been abruptly closed by the remote host". The server is assigned 8 GB of RAM, my operator is netia, mostly orange, the firewall is turned off and the server itself rarely lags even with a large number of players. I will be grateful for any advice that will help my colleague

I will add that it is rather not the fault of my colleagues' equipment because one has an old crap that barely reaches 60 fps in the game and the other is in the witcher at the highest without clipping

Apparently they have unstable internet since they are disconnecting (your server says so and rather knows who ended the connection). Maybe let them try to use another internet for a test, e.g. from a telephone. This does not have to be the fault of the computer itself, the problem may be on the side of their net provider, router, WiFi ... you need to test a little