The root cause of bug #239038 appears to be that protobuf compilation (no longer?) includes pthread in the way that is needed for net/mosh to function. Compare the error message:
[libprotobuf FATAL google/protobuf/generated_message_util.cc:809] CHECK failed: (scc->visit_status.load(std::memory_order_relaxed)) == (SCCInfoBase::kRunning):
to the one reported in https://stackoverflow.com/questions/52890529/fatalexception-thrown-by-protobuf-library-on-instantiation-of-message:
[libprotobuf FATAL /myproj/protobuf/src/google/protobuf/generated_message_util.cc:785] CHECK failed: (scc->visit_status.load(std::memory_order_relaxed)) == (SCCInfoBase::kRunning):
In that stackoverflow discussion, the solution is to add "--no-as-needed" before "-pthread" to the linker flags for protobuf.
This suggests that a similar tweak to the protobuf build instructions in FreeBSD ports could also help. I am not sufficiently familiar with ports to try this myself.
Since all the information, analysis and a patch is in bug 239038, which has now been re-triaged and re-assigned to the devel/protobuf maintainer, close this as a dupe
*** This bug has been marked as a duplicate of bug 239038 ***