thanks a lot for clarification about hostname. it was the hostname setting in the vpn-dashboard, not the settings in hestiacp.
i was now able to purge the cache in the wordpress settings.
regarding cloudflare, what do you mean it blocks by default?
i would go to setup cloudflare similar like @desp has described his tweak of hestia template file here, so that cloudflare will cache the files. do you think that this will interfere with the nginx wordpress cache plugin?