It is likely random whether the server works or not without
a manually configured protocol option, which is neither documented
nor enforced to be set.
With a random uninitialized value in m_protocol, this can happen:
[2025-08-03T12:39:57] NOTE: accepted client connection
[2025-08-03T12:39:57] FATAL: a runtime error occurred: XInvalidProtocol
Reports of "it works with debug enabled" are likely due to having
parts of the memory space the server object is allocated into being
cleared, and thus having a 0 (Synergy) startup value.
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