The error indicates that the very first attempt to communicate a question from the library to Poser itself (main program has an API reached via HTTP) failed.

Both the embedded and external versions use this API to talk to the main program. (Which works also with other SM apps like Anime Studio, not just Poser.)

This rarely means Poser isn't running, although that can be a cause, but not likely in your case.

The more common is a firewall blocking the connection. This can be blocked by at the client app or at the server app or both. It can be blocked for all I/O or just local. (We need local and no actual access to an attached network - it's loopback.)

Now since you report the embedded library works, it hints that nothing blocks the server side. Something is blocking LSMGUIAIR.exe itself. 

This is not certain, but the reported facts imply it.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)