Steps you can take to reproduce the problem:
- Create a new task in Planner
- Assign it to yourself
- Go to Outlook and find the group and the task you created which will be listed as a conversation.
- Delete the conversation (it will notify you that the action is permanent)
- Now go back to Planner and try post a new comment. You should get the following error: "We were not able to post your comment. Please try again"
- Then, if you close the task, browse around a bit then re-open it in a while you should start seeing the error message in the title of this post.
Hope this is useful to others.