yer check the Bugs forum for a topic on this, it appears that its happened a couple of times this season to other users.
So yer, just to confirm, there is no user ability to be able to 'backout' of a scrimmage. If you are booking a scrimmage sucessfully and it appears in your schedule list, take a screen cap. And if the scrimmage disappears, post it in the bugs forum and ask Marin to fix it.
Unfortunately its a bug that does appear to have manifested itself this season.