the tftp client cannot send packets properly

dear all:
hello, I’m sorry to take up your time
I am using the w5100s chip and after configuring the network, create one of the sockets as the tftp client. Sometimes I can use it to get files from the server normally. However, if the device is not powered on for a long time (one or two days), files may fail to be obtained occasionally. In this case, I used the packet capture software and found that no request packet was sent at all. Print to find that the snd len returned by sendto is negative. I wonder why this is happening and how to solve it.
And the amazing thing is that when I restart the upper computer this goes away.

send error:

After the restart: