Okay, I had the same error again just a moment ago with another question, and I figured it out: I opened a question, then I clicked on Improve. While I had some delay between those two actions (phone rang), somebody else clicked on Improve before I did. Since I didn't refresh the page yet, I didn't see, that the question is locked by another member already. Instead I get the error message, that I posted in my original post.
"I love deadlines. I like the whooshing sound they make as they fly by." (DNA)