Google My Company create a post error

Hi, we are automating creating posts for our Google My Business locations, where we load the image from our Dropbox account. It goes well with a few dozen scenarios, but we encountered a weird error in one of them.
We use the create/update a share link module in dropbox, and then we use the share link in Google My Business, but it creates an error:[400] Request contains an invalid argument. Image format is not supported.
When we add the same picture directly into the new post, it works well. The image should meet the Google standards, its jpg, 30KB, 618x456px. I know that the recommended resolition is 720x720px - does somehow block smaller pictures?