Issues with moving posts and images

Hi all,

I am having issues moving posts and images from one WordPress install to another. I am exporting posts in a certain period using standard WordPress functionality. Then I am importing them to a new WordPress install in a subdomain. The posts are boing moved OK but:
1. Random images are becoming the featured images of the imported posts. Completely random. And those random featured images are linked to images already in the server.
2. Import media files option is not working. Neither when I import posts not when importing media only.

Both installs are WordPress 4.9.9.

What could be the reason for these issues? Thanks!