Is that correct, that my custom export plugin gets 'exSelBeginInstance' selector twice?
Try better explain:
Premiere run two instances of my exporter. It easy to see by looking at exporterPluginID value. It different. And at second instance exporter doesn't get selector 'exSelGenerateDefaultParams'.
Premiere and AME open the plugin multiple times.
If you put a breakpoint on the selector you'll find that it most likely also called exSelEndInstance between them.
here are the basic steps.
If you do that you'll be thread safe (AME runs multiple instances of the plugin at once).