I posted 3 pictures yesterday and tried to do an4th today, but I got this warning. It always does this to me! The most I've ever been able to post is 5, and then it refuses to let me post that last photo. All a bit strange really.
It's on the app (must be the latest version as they all update automatically), on iPhone 7 using iOS 10.3.3.
It's been like this for about a year now.