This is expected behavior due to a thread-safety fix in POCO 1.14.0 (commit ece3603, PR #4512, issue #4435).

What changed: A mutex now protects sendBytes(), receiveBytes(), and other SSL operations because OpenSSL SSL object is not thread-safe.

Why it hangs: If one thread blocks in receiveBytes() waiting for data, another thread calling sendBytes() will block waiting for the mutex.

Solutions:

1. Use non-blocking sockets with poll/select
2. Add receive timeout so the mutex is periodically released
3. Use single-threaded I/O with event-driven design (SocketReactor)

The previous behavior (no mutex) was unsafe and could cause SSL errors during TLS renegotiation.