I've had this problem a few times, when some of the messages I write or forum posts are blank. I've given a couple examples below and have indicated with strikethrough text the items which come up blank when you send the mail/post on the forum.
Typically this is when I try to suggest a route to haul with trains, or if I post a link:
MAIL EXAMPLE - a message containing a suggested route like this often has the route coming up blank when you send it:
FORUM EXAMPLE - posts with links. The second link in the post below is always blank on the forum when I post it.