Happened to me as well now. Has nothing to do with a speed restriction on a specific waypoint, but i deleted the 250/10000 constraint (got a free speed from ATC) and then lost all my VNAV-data. EVen reentering a new Cruise level didnt work.
Works without a problem on a 737NG (NGX).
PS: I got my VNAV back by changing the approach (both STAR and approach).