In the end, I had to track this when I came across it myself. The following is written on the manual page:
connreset: The number of times a TCP connection failed due to a RESET from the
server. Typically, a RESET is received when the client attempts to send data to
the server at a time the server has already closed its end of the connection.
NT servers also send RESETs when attempting to establish a new connection when
the listen queue is full.
In order for my own server implementation to stop doing this, I had to disconnect the connection on the server side. It turned out that the server is too aggressive towards closing the connection.
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