I would first check that the paper is square. Paper arrives out of square more often than it should, and the 3800 is even more picky about that than the 3880.
Also, if you haven't used the manual rear feed for a while, you may be out of practice. You have to release the paper as soon as the printer grabs it. If you delay, for even a second, it will through an error. There is a definite knack to using the rear feed.
Setting the paper thickness to 15 (1.5 mm) is not a good idea - it is as likely as anything to cause missfeed errors. There is, however, a widest platen gap setting. It is only available from the printer console menus. It is one of the few on-printer settings that overrides the driver settings.
Brian A