Hi @anton_pelayo,
Thanks for reaching out.
There might be a few different reasons for this issue, I would suggest following troubleshooting about your demo import.
1.) Increase PHP Memory Limit
2.) Increase PHP Execution Time Limit
3) Please check if the mod_security is enabled in your site, which causes the issue in importing and does not let you call the Cornerstone endpoint to import the Design Cloud assets.
And if that is the case, you need to contact your hosting service provider to temporarily disable the ModSecurity for you and make the Cornerstone endpoints whitelisted.
4) The write permission to the uploads folder might be one of the reasons behind your issue, I would suggest you change the permission to import the images into it.
I also recommend you ask your web host to configure the following server settings with the specified values or more for the PHP.
memory_limit = 512M
upload_max_filesize = 512M
post_max_size = 512M
max_execution_time = 300
max_input_time = 300
max_allowed_packet = 16Mb
innodb_log_file_size = 128mb
allow_url_fopen = On
And MySQL Configurations
max_allowed_packet = 16Mb
innodb_log_file_size = 128mb
Hope it helps.
Thanks