assertion "unsent_oversize mismatch (pcb->unsent is NULL)" failed:

[BlackEdder]

Every so often (once every couple of hours) tcp_write fails with: assertion "unsent_oversize mismatch (pcb->unsent is NULL)" failed. The same code works fine on my ESP8266 boards.

[ESP_igrr]

tcp_write can only be called from LwIPs own task, as raw API is not thread-safe.
Calling raw API from the application used to work with ESP8266 non-OS SDK, which was single threaded. But FreeRTOS is not.

[BlackEdder]

Thank you, I have now been able to solve the issue!