FB group posting - image problem

Hi, I have page which redirect to other page. It's simple page where are only og tags like description, title, type and most important og:image where is link to dropbox image (I try a lot of upload services = same result).

I'm posting manualy to FB groups. If I post to 10 - 20 groups, Facebook block URL because of image and link is deleted from every group. Message:

"The content you're trying to share includes a link that our security systems detected to be unsafe:

(link on dropbox image)

Please remove this link to continue.
If you think you're seeing this by mistake, please let us know."

Now it's happend almost everytime. Before two-three weeks ago I don't have this problem.

Do you have solution please?

Sorry for my english.