First editing of a post (error message)

[sofos]

Hello, I am having a bit of a nightmare today.

Could you please help me with another issue?

When members try to edit their posts, they get an error message at first (see image attached) that says that they can't edit the post. Then at the second try they can edit the post and the editing appears fine. Why do they get this first error message? 

Thank you so much for your help!