Hi,
Seems like server blocking import files transfer. When such thing happening simple way is to install everything locally or other server and then migrate with all in one import or similar plugin to your server.
Version 2 here for migration instructions: https://wpdirectorykit.com/documentation/#migration
Please contact us via official website and also provide your server ip https://www.ads-software.com/plugins/real-wan-server-ip/ and which theme demo data you try to import, so we can check your case more in detail.
Thanks