Block thumbnails not displaying

Questions
1

Earlier this week we went from a single production server solution to a multi server production solution. Now we’re having trouble getting the block thumbnails to be displayed.

Backoffice is accessed on one of the servers only and by a sub domain address. It’s blocked only allowing calls from a few ip numbers.

How can we get the thumbnails to work?

I read here something about whitelisting the server but I don’t get what I need to do. How to debug the "new" block thumbnails - #13 by NilsN

Litium version: 7.7.8

2

Is the blockthumbnal folder contains images? Is it accessible for all servers?

3

Thought I had written here. It seems that it solved itself. When making a change in a block on the page and then saving it the thumbnails started to show up again.