I don’t want to appear to be a donkey here, but, have you compiled the code with the 4.9.1 branch?
The reason I ask, is I have no doubt that the modules that load the plugins are looking for something, that has to be present in the code prior to the actual loader starting, immediately after, or at some point, and is dumping this message because what ever the engine is looking for it’s not finding.
This definitely happens with blueprints, and I see no reason why Epic wouldn’t protect the engine as well in terms of loading external code, outside of their control. I sure as heck know I would.
Just my thoughts,
Have a great day!