I have a simple test, that does the following (netnext here is server, mptcp.org is client):

server-side:

fd = socket()
bind(...)
listen(fd, 5)
sleep(5)
close(fd)

client-side:

fd = socket()
connect()
write(fd, ..., 1)
sleep(10)
close(fd)

First I launch the server-program, wait 1 second and then the client.

This triggers a IPv4: Attempt to release TCP socket in state 1 00000000cbc88a4a