I’m clicking the Flag button at the bottom of a closed topic, choosing “Something else” and then entering in a brief reason why I’m flagging the topic. (in this case, suggesting that it be re-opened as the initial problem is back)
I flagged that post as TL1 user, chose “Something Else” entered the message and I was successfully able to do so. The post was flagged and new message was created.
Okay, I can repro the issue. I was clicking on flag icon below the post instead of the Flag button at the bottom of a topic.