When client disconnect the send function throws an exception in 'cancel_timeout()' - 'std::unique_lockstd::mutex lock(timer_mutex);' row.
The exception is:
"Exception thrown at 0x09643473 (msvcp140d.dll) in mtsrv.dev.exe: 0xC0000005: Access violation reading location 0xDDDDDDDD."

This happens when client disconnected at the same time when server is sending data.