Thanks for this solution.

I was also searching for answers by the FR community, and here is how i managed to get out of the problem, getting back the post.

1/ The problem seems to happen less (not at all?) when the auto-connexion is on.
This can be activated under 4.8 -> 4.9.3 by changing an option in the "default.aspx" file :
search for autocomplete and set it to on

2/ To get all posts again, you can try using those 2 SQL command lines.
The first will show you all posts that were posted by "anonymous" (means "out of time" if anonymous is not accepted)
           SELECT * FROM forum_posts WHERE UserID = 0
Keep all the numbers under the "PostID" column.

The second will correct one line. You'll need to execute it for each "PostID" you kept in the previous step.
           UPDATE forum_posts SET UserID = X1 WHERE PostID = X2
X1 must be replaced by the user ID that posted the answer (this ID can be seen under "user" admin page, when looking at the details of a user)
X2 must be replaced by the "PostID" you had.

Maybe someone will make a small SQL script that will create a "backup user" and then give him all lost posts, i'm not sure enough of my SQL skills for that.