Hello,
I had the same problem and it was driving me mad after I have changed to new a theme last week. I have called theme and host server teams to help. No success.
I know little about WP configuration, etc, etc. I just followed an instinctive suggestion: deactivate the plugins one by one. I started from the one which has to do with images. In my case it was an plugin for image compression. BINGO!!
The grid has come back!
I hope this helps. It is really annoying gys.
tom
