Problems exporting/importing entries/media

I want to play around with making a new design for a site. This new site will contain content for two different sites A and B. I’ve set up a password protected site (htpassword) to play with things.

I can import all the entries from site A without problems (short entries, text only) but when I try with site B (longer entries, text and photos) two things happens:

+ I get the error message below which is repeated a number of times

Warning: mysqli_real_escape_string() expects parameter 2 to be string, object given in /xxxx/xxx/xxxx/public_html/wp-includes/wp-db.php on line 1171

+ No media is imported

When I exported from site B I only exported entries (all of them) and when I imported I checked the download media (or whatever it says) checkbox.

So this means that entries that used to have “featured image” now has none.

I assume I’m doing something wrong but what?

(on B I have a few plugins active that I don’t have active on my test site: loco translate, nextgen gallery, page builder by site origin – but as far as I know I’m not not using any one of them for the entries I’m exporting)