Program chrashes under binary file upload

Hello,

I'm new in this forum and in this area. Please help me some advice if it is possible. So I'm working on a webserver application, and I use the lwip 1.4 tcp/ip stack. I implemented the file upload, but sometimes when I try to upload the files, get TCP retransmission and TCP Dup Ack error on network capture. Sometimes the file upload was succesfully and somtimes wasn't. I didn't find the right solution up to now.

The two captures(one when the upload was succesfully, one when wasn't) is available at this link.

https://we.tl/BYayHqMAEV

Please use the following filter to see the captures between my PC and my device: ip.addr == 192.168.0.23

Please help me. Kind regards, Szabolcs