Failure to load plugin

I’m working with the low level API (latest 1.x).
I try to create a new output plugin (based on the output_mp3 example).
When I tried to load the new plugin I got FMOD_ERR_FILE_NOTFOUND error.
When I try to load the output_mp3 plugin which is in the same folder, it works OK, so it is not a matter of wrong path set by setPluginPath.
I guess there is something wrong with the .dll I created.
What other reasons can make loadPlugin return FMOD_ERR_FILE_NOTFOUND?

Most platforms give file_not_found for all plugin load failures, so file not found, access denied or wrong architecture. Windows is the exception and should give more specific errors.