When something is closed for years and we don't use it, and after years when we open it, there are a lot of changes and a lot of new material, that's why we are asked for authentication if properly authenticated. then this problem is solved, sometimes it happens that old things cannot be opened or sometimes it also happens that  Your account should be permanently disabled.

Because accounts that are not opened and used for years are put on the list of dead accounts and many are closed when trying to re-open and do not dissolve. Same with Google accounts. The problem is the same with Facebook accounts and accounts on other sites that become inactive after being closed for years.

So, it's possible that the message or old you are trying to post in belongs to a dead account that has been deactivated, hence the post is only visible but it can't be updated anymore.