Thank you very much Remy for your explanation!

I checked the communication between the server and the client through Wireshark. All packages were send without errors and the server sends the expected number of bytes (no expections).

Nevertheless, I somehow found an alternative solution to avoid these timeouts by sending less bytes within one data stream. So the issue was on the serverside. Before, I was sending 135172 bytes at once. Now I send 20484 bytes at once.

I seems, that the TCPWindow was too long.

I was using (and still use) the settings recommend here: RTOS Settings