Are you sure you are building with the CC 2018 SDK headers?
Up to CC 2017, Illustrator would load plugins built with the original Illustrator CC SDK or later. For some reason Adobe broke backward compatibility in CC 2018 when the version number for the AIFilePath suite increased and CC 2018 refused to load earlier versions of this suite. Version numbers of other suites have changed since the original CC version and later Illustrator CC versions still load the older suite versions.
You may not think you are using the AIFilePath suite, but if you are using the common code for your plugin, Suites.cpp will try to load the AIFilePath suite and if it fails you will get the error loading plugin message.
As you are working on Windows, the other possibility is that you are building a 32 bit plugin for a 64 bit version of Illustrator.
Thank you very much. Changing the test plug in to 64bit did the job.