Thumbnail half invisible

Hi,

I have problems using the grid. Sometimes, the thumbnails are showing fine, sometimes I see only half of them. When looking at the css in the browser, I can see that the image is there, but half of its height is invisible due to

div.grid-stack-item.ui-draggable.ui-resizable.ui-resizable-autohide.ui-draggable-disabled.ui-resizable-disabled

which is sometimes set to 258px like it should but sometimes set to only 74px; If I refresh I get either one or the other.

I took a few screenshots but I can’t include them here.
Thanks