Search bar for adding related products doesn't work

[Ana MBL]

We've updated the shop to version 1.7.8.7, and now we have a problem when editing a product in backoffice. The search bar which lets you add related products doesn't work anymore, nothing happens when you type something there. So now it's impossible to add related products.

The products are indexed, and the front page search bar (on the main page of the shop) works perfectly. The problem is just with the related products search in the backoffice. There are no errors in the logs, and when I look at the network tab in developer tools there are no requests sent when I type something in, so it seems it's just not doing anything.