[@johnnyontheweb Participant 6 years, 10 months ago]

Thanks to all – I solved the problem. To be useful to others I report a some hints:
inexplicably, some capabilities in the Subscriber role in my wp installation were missing (publish_documents and, but not sure, override_document_lock) – I had to use Profile builder plugin to clearly see what’s missing, User Roles Editor was too much confusing.