ExtractPDFOperation.Execute(...) hangs when run from under IIS

I am trying the sample provided with the SDK. It works fine when run within a console app, but the very same code snippet hangs called from a back-end code.

I can see that the SDK successfully makes a call to https://pdf-services.adobe.io/assets, gets uploadUri, but never uploads the asset. I can confirm it is not an issue of access rights to the folder.

I have run out of ideas about what could be the possible cause.