I had the same problem when I updated my DIR-655 A4 from 1.35NA to 1.37NA. The first time around I must have gotten lucky, because the router eventually accepted the file after repeated tries. Subsequently I had to go back to 1.35NA in order to troubleshoot a network problem I hadn't noticed until after the update. I eventually determined that it had nothing to do with the firmware update. When I updated the router to 1.37NA the second time, I ran into the same problem. However, for some reason I looked at the router log right after the file had been rejected, and it actually said why (imagine that). It turned out that one of the reserved IP addresses was in use by (i.e., had been assigned to) the computer I was using to do the firmware update. As soon as I configured the computer with an IP static address that wasn't reserved, the router accepted the configuration file. Your problem may be something different, but at least the log might point you in the right direction. Good luck.
Larry ....