It took some time to find the answer in the help forums i.e. there is a work around until this problem has been fixed - therefore I am posting this here in case you are desperately looking for a solution as well:
Go into the "edit" mode of your post, look for the first image that will normally appear as thumbnail and replace the following:
"The solution lies in replacing https with http and googleusercontent with ggphtSo for example:https://lh3.googleusercontent.
Be sure to do this twice in the code of your first image - otherwise it will not work!If you do these substitutions in your first image, the thumbnail will be generated without problem."